I think it depends on the particular pill. Check the package insert or call your pharmacist to be sure. Certain pills are very, very sensitive, but I think those aren't as commonly prescribed.
Personally I give myself a 1-2 hour window and haven't gotten pregnant. I do my best to keep it in the one hour though, I actually have a specific alarm for it. Although sometimes that's difficult when I'm not home or am traveling, which is when I use the 2 hour window. Personally, I would brush off a few hours. However, I'm not very concerned about getting pregnant (It would just be very bad timing). So you personally need to weigh up the risk.