I would arrive around a few minutes either side of 6:00 if the party were being given by someone I didn't see regularly.
I have one friend who entertains a lot and likes us to show up earlier than the time she gives. If she says 2:00 and you arrive there at 2:00, she'll scold you for being late. That actually happened with her this past 4th of July.
I also have relatives who are always running behind and will leave you waiting for hours while they dress, finish cooking, setting up furniture, etc. We try to help, but they tend to be disorganized, it's not so easy to do.
So I guess the answer is that it depends on who invited us.