I use half inch at least. For silver contacts, my logic is that you want the top surface as far away from the print as possible, so that if there's any dust or surface defect on the glass it'll be diffused out, i.e., you only have to worry about the surface contacting the neg. Plus the added weight makes for some very nice flatness even with the matte papers I prefer. For alt contacts, the transmission may be an issue- a lot of glass cuts out at ~350nm. But it hasn't been an issue for me. So I say go with thick. If you need to do a lot of dodge and burn then you may want thinner so that you can control the light a bit more precisely.