I think it should depend on the type of art it is. Basically, as a general rule: It shouldn't take too much longer to download something than it takes to look at it... or something like that.

Ergo, simple B&W sketches should probably be grouped together in sketchdumps (the simpler the art, the bigger the dump). If they're elaborate and/or colored and/or not related at all to each other, it's probably better to put them up separately.

That's my opinion, at least.

Personally, I think it's better to have 1-5 sketched commissions in a batch rather then submitting all individual pieces. It helps to avoid clutter in your gallery, however, I do recommend submitting separately in your scraps so they can have the option to fav the ones they reeeeally liked. *shrugs* I suggest going with a batch because I know I'd go nuts if I had to upload every single one to my gallery! xD

Haha, everyone wants to make you work harder. xD
I say it depends. If they are just simply sketches or inked lines, then it's fine to have them all clumped together, but if they are more in-depth pieces, they should be individual.