I keep a bag of the sizes that I mainly use, which have each had a couple of plane strokes all round to slim them down. They're all marked with a big red "X" on each face so they never get mixed up, these are the ones that get used for dry test fits.
Other workshops I've been in sometimes kiss the domino on a sander if a job will require multiple test assemblies. I know craftsmen way above my level who follow this practise, but personally I'm not convinced. I trust in Festool to have optimised the fit for the strongest possible joint, so I prefer to do the final glue up with untouched, full size Dominos.
On the subject of tight Dominos, if you glue up a big, complex frame with Dominos and PVA you can get caught out by the PVA "grabbing". On a hot day, especially if the Dominos have swollen slightly because they've not been stored properly in the dry, you can get the joints 90% home, but if you'd messed around too long when you get the cramps on for the final tighten the glue has "grabbed" (PVA is pressure sensitive) and either won't shift at all or needs a big sash cramp with a pipe on the tommy bar. To avoid those kind of heart attack moments I take real care storing Dominos to make sure they're bone dry, and on really complicated assemblies I'll us a slow setting UF glue (such as Cascamite) which isn't pressure sensitive.
One final point, on the really big XL Dominos the fit seems to be a fraction looser. Just the impression I get, maybe sample variation, be interesting to hear from other users.