From my point of view, a small appreciation (for heng heng) to PG, VG & MUA will do cos we already paid quite alot for their service already.
And if u have spare seats, i tink it should be fine getting them to sit if they needed to. But most of the time, they will only need water as they will be following everywhere the bride and groom goes.
For my own case, as my 2 PGs are my own frens, i would most probably provide them an early dinner and enough time to rest. And they only requested to have water during the banquet, so i suppose they will be walking around for shots as i wanted journalistic style rather than portrait styles pictures.