Engrams don't go into the postmaster until the completion of the activity. I tend to use my PM to hold infusion fuel, and some weapons that are good enough to hold onto, but not vault worthy... so at times I overflow. I try to pick up every engram that I see to delete because of that myself... I would definitely like to have PM access anywhere, but because the collection occurs on activity completion, you would still be in a similar situation where you can overflow when completing an activity. Ideally, you just want to keep it empty, in my case, if i overflow I don't really lose much other than infusion fuel most of the time, which is easy to come by once you got pinnacle cap.