We have been down this road with our younger daughter. We're a family of giants (yes, really, DH and I are 6' tall+ and our 7-year old is 4'8", if that tells you anything). Our little one is LITTLE. Like, the size of a small-ish 3 year old, but she's turning 5 this month. She was normal sized when she was born, and pretty normal up until about 6 months, and then at her 9 and 12 month visits, she slid down to the 15% for height and 25% for weight. We thought this was pretty weird. Pedi said to wait it out, maybe she'd hit a growth spurt. So... we waited until she turned 4. Surprise, still tiny. So we went in and had a million tests run at Children's Hospital. They came back completely normal. No problems with this kid at all. She's just little. Of course, after having been stuck with a needle for about 20 different tests, she is TERRIFIED of going to the doctor now. We pull into the parking lot and she starts shaking and her eyes fill with tears. It's awful.
So in my opinion, if your baby has rosy cheeks, is alert, happy and is hitting all the developmental milestones, I'd put off the tests. Not every child is going to be in the 95% (because... if they were, it would be the 50%, right? =) Some kids are above average height and weight, and some are below average. Now, if your doctor sees other issues besides height and weight (for instance, your baby has big dark circles under the eyes, or throws up every day, or whatever), then get some testing done. But if there are no other problems, don't worry about it. No need to make your baby hate going to to the doctor if you can avoid it!