Hi A., (sorry, looking down I realize now my answer is quite long! I think it matches the stress we've gone through with this issue!!)
Oh the seams, the threads, the tags, the zippers, the belts! Yes, I would have to spend 45 minutes each morning putting on my crying daughter's socks while I was later and later to work. So, yes, I know what you mean! My daughter is now 14 and barely has any signs of it (but still definitely prefers soft clothes. And my son who is now a preschooler could barely be potty-trained because the smell of his own urine coming out made him gag and he would cry each time! How many times did I have to tell him it is rude to meet someone new and say, "What's that smell?" Geeeze!
And let me just insert here: my kids are normal in every other way...don't show signs of autism or any other disorder. Yet, I really took this issue seriously since it can be a symptom of bigger problems, and it is important to address these issues when the kids are young to help their brains and senses develope the best they can. What mom wouldn't to that, right?
Okay, so, how did we get through it? Here's just a few thoughts:
1) Believe and validate. She really IS feeling like her toes are being dragged on jagged pavement by that horrible seam in her socks, even though it seems outlandish to people. I learned this when I tied a bow on the back of my daughter's Christmas dress for a party when she was four. My daughter (very compliantly) sat at a table for about 45 whole minutes. Then with panted breath, she walked slowly over to me and said, "Mama, I have a shark biting me in the back." I got the scissors out and 'snip, snip' off the belt came.
2) Ignore (totally) the advice from those who don't understand that says something like, "Just expose them to it. MAKE her wear the tights and she will get used to them! It's your fault for not doing it when she was really young." Right! If they only heard the screaming, right?
3) Socks! We put the socks on INSIDE OUT. That way the seam was sticking out instead of sticking in touching her toes!! But then there were the very small strings that hang from either side of the seam. So, we cut those as short as possibe without putting a hole in the sock (can you imagine your daughter's toe sticking through a hole and her managing that? Ha, right?) My daugher would sit for 1/2 hour putting the sides of the seam equidistant from each side of her foot before she could put on her shoes. But it gave her something she could SEE. And SHE did it; I was free to continue getting ready or whatever. No more frustrated yelling matches at the door! We also bought the "gold toe" socks (they are just gold across the toes, found them at JCPenney, I think Target has them, too). They are super, super soft (yes, be one of those people who rips open the package to feel them to make sure!) Also, her toes were covered in gold! Kinda psyched her out a bit, ya know?
4) I don't know if you are into this sort of thing or not but... we used thrift stores alot. The clothes there have already been washed several times and so they are softer. What you try on is what you get. No more, "Well, it looks great on you, honey. And don't worry about that itch seam/scratchy tag. By the time we get it home and wash it a couple of times, it will feel great." ... just to have it sit in her closet forever! Much, much easier!
5) Occupational therapists deal with this sort of thing, if you would like to go that route (now or later). The issue often thought of is called "sensory integration dysfunction" and has to do with how your daughter's brain processes sensory information, either too enhanced or too subdued. There are lots of websites one of which is http://www.incrediblehorizons.com/sensory-integration.htm
6) One occupational therapist had some great advice: Take a brush (VERY soft, she recommended a surgeon's brush that is used to scrub their hands...it is much like a vegetable cleaning brush but softer...and you can get them from occupational therapists or hospitals) and gently brush your daughter's skin with it. Believe it or not, my daughter found ours before I had a chance to try it. She immediately picked it up and brushed her arm with it to see what it felt like and said, "Oh, mama! Can I have this?" You could see in her whole body the relaxation and relief. That was the beginning of the end for us. What it does is (from what I understand) sort of resets the body's neural interpretation of touch. It literally changes how the skin feels things. We (or she) used the brush every day. Later, we used it before ballet ("Here honey, let's get your skin ready...") and she was able to wear tights!!
7) We bought one of those tunnels kids can climb through (they look like big caterpillars) and both my kids 'played' with that. The benefit comes from the right/left motion of the body that 'crosses the midline' when crawling; it integrates the left/right sides of the brain in such a way that they are more able to handle external sensory information.
8) If the smell is a problem also, we lit candles in the bathroom for our son to go potty. Worked great.
Okay, sorry so long! Hope this helps...