I went to a wedding a few weeks ago and the invite read 5pm so we got there at 5:20. We missed half the ceremony! Of course we felt bad and the truth is the couple didn't notice, but we were surprised that the wedding actually started at 5pm on the dot. All of the weddings we've ever been to started about 30 minutes after the invitation time. During that half hour guests will mingle, have some water or champagne and find a seat for the ceremony.
Talk to your ceremony location regarding exactly when the ceremony will begin. Does it have to start on the dot because there's another wedding behind yours or is there some wiggle room? And then talk to your parents and his -- they'll know if their relatives and friends are used to a window of time before the ceremony or if they're used to a wedding starting exactly at the time listed on the invitation.
I remember a wedding in college that started over an hour late because most of the guests didn't arrive until an hour after the invite time. And then there was another wedding where the bride knew her guests had a tendency to arrive late by an hour or more so she added the word "promptly" to the invitation time so those guests would arrive on time. It worked.
