I agree with you entirely that once a dog does a stay it is neccessary to proof it.  In our advanced classes we return past our dogs, walk over them, have the instructor walk around them, all handlers go for a walk around the other dogs at the same time.   
In a trial I don't see any point in pushing your luck but training should be about finding and extending the limits. 
If obedience is getting a bit of a bore, keep it up but give agility a go as well.  It is great fun for you and your dog if you don't take yourself too seriously.