I hated poeple who did this...lol. It got right on my tits, all those labels they stuck to classic tuneage - like a big fek off white label with '122' daubed over it or the vinyl / cover in marker pen.
What you can do, if you wanna do it, is either by one of those 'cheat' ( imo lol) beat counters for your mixer or play your vinyl on the absolute pefrect zero on your stroboscope (blips) on your platter and play fruityloops adjusting the tempo until it matches. That way, the absolute dead speed of the record is kept so you know its initial value.
I suppose it helps knowing the initial speed, but you should know by instinct whether a tunes well fast or not, ie dont mix Hixxy and Dougal into black box....or CeCe Rogers into DjTrix etc....lol.
sirius.