so im reading this book on rebuilding the sbc and it says some people grind a 30 degree back cut on thier valves just under the seat contact position to improve flow, is this true?

Yes.

Pretty common too. You can buy them pre-done, look on summit at valve for an SBC, they'll have "backcut valves".

You can get your machinist to do your stock ones, but if a few are bent or worn on the stems, might as well get a new set that's already done.

I did it by hand, do a search under my name and "home back cut valves" or something, if you're interested.