I tend to go both ways with my canopies. I usually have the cockpit open if there is good detail, and even if there isn't I think it just looks more interesting. However, anytime that I have crew members, the canopy is usually closed...for obvious reasons. But there are those models whose canopies have come off with the passage of time and it is often difficult to reattach them in the open position... so those are all closed now.
I usually use Elmers white glue for my canopies, but it it needs extra strength, I will use a conservative amount of plastic cement. If used properly you can avoid frosting, but it usually isn't worth the risk.