Perfectly allright! Done here quite a lot, I leave the BIAB bag to drain overnight for the dregs, same with kettle trub, then freeze any liquor or wort if I'm not about to boil & use it in a starter.
If I've got strong runnings out of a dunk sparge I'll often resparge the mash with a litre or two and collect that for yeast starters. Waste not, want not! :icon_cheers: