I give more bday/kids parties than dinner parties, but if I put 6pm, between 5:45-6:15 would be perfect. Any later than that and I think its rude since tardiness is a huge pet peeve of mine. At most parties I have or have been to and everyone is usually 15 min early or late. I have a set of relatives who are always 1 hr late, if the party is 1 1/2-2 hrs they missed most of it, including food. I don't get being late.
Okay, I probably host more kids birthday parties (out) than dinner parties, but they last no longer than 2 hours. However, the adult invited parties will last up to 6 hours, so being 15 minutes late is normal. I honestly don't understand how some people think it's okay to be early, but rude to be late. At least around here, being early is considered very rude, but being a little late is the norm. And the most fun part of parties is before and after the meal.