It's whatever you're comfortable with doing. If you're not comfortable giving it a little early, then don't do it. Just because other nurses do, doesn't make it right. I always do what I feel is right and what's accepted nursing practice. However, I do give PRN's a little early, as well....depending on patient assessment, and what the med is. If it's a prn zofran, yes, I'll give that 15 mins early. Nausea/vomiting is an awful feeling, and if it makes my patient more comfortable, I'll give it. IV phenergan early? and they're a little old lady? probably not...i'll do ginger-ale or something first. If it's prn dilaudid, and they just had 1 mg 2 hours ago, then they're waiting until the 2 hr mark; that's too close there. If it's however, a PRN lortab, and the person's been taking them at home for 15 years lol, and it's every 4 hours, I'll give that 10 mins early. If I find that my patient continues to want their pain meds 1 hr earlier than they should (which I never give them that early....only like 10-15 mins, and they really seem like they're in pain, I call the doc to change the frequency).
I don't work in peds, though. I work with adults, so I doubt you have any long-term chronic pain med takers on your unit lol. Or I guess you could if they have cancer or something, but it has to be more rare than the adult population.