I think the bigger question is when you first let them go to a house where you don't already know the parents. I tagged along to one play date in year 1, and was very glad I did (no danger, but having been promised lunch, it wasn't until about 2.30 and a very odd lunch - obviously I couldn't do much about it, but I was glad I knew, and if daughter had got tetchy because she was hungry, I could have intervened).
Of course, if you don't do the school run, it's harder to get to know other parents - Tigerbalmshark's suggestion is good for that.