I guess the question is define "tall." If your child is freakishly large, like 4 feet tall at age 3, then you'll probably be asked. If he/she is just a bit tall for an almost 3 year old, then you absolutely don't have to buy a ticket.
We have tall-for-their-age kids in my family--four kids in total currently going from age 8 to age 3 who are amongst the tallest in their classes/amongst similar aged peers. We've been to Disney parks at least once a year since these kids were babies and never was stopped, questioned, required to show kid's birth certificate, etc upon entering any park. Basically, if your kid can pass for under 3 years of age, even if he/she is a bit large for that age, Disney doesn't care. Hell, if your kid is small and is above the age of 3 (can pass for young), they don't care. I've never seen anyone stopped and/or required to purchase an extra ticket.
I think it's always been a "don't ask/don't tell" courtesy situation. And since your kid won't be 3 until after your trip, why should you feel guilty? Save your $$, and if you still feel guilty, then buy extra goodies and souviniers.