We had quite a bit of trouble about 18 months ago with stamps coming off. We found it seemed to be due to the small stamps that come on rolls. Do we quit buying those. Now we only buy the large format ones that come in booklets of 400 as they seem to have better glue on them. Like DG the recent lighthouse stamps seem to have the best sticking power we have seen.
As far as putting tape over the stamps, it was our own local PO who recommended that we do that when we were having trouble. So I think it very much depends who you get.
We've not had any trouble with underpaid postage for quite a while now. We've not been using tape and all our large letters go into our local red street box.