I've been given some 1990 series $100 bills & am wondering if I'll still be able to use them around the place or do they have to be replaced with the newer style bill?
Yeah, you'll be fine. Some machines might not like them but most humans are capable of recognizing a real - if old - bill.
If they work, great. If they don't work, replace with newer bills. If you're worried replace with newer bills at home before you hit vegas. Nothing to fret over.
By the way, I have found in some cases that OLD machines are more tolerant of older bills. I have used them and then immediately cash out with a TITO. Okay, maybe I give one spin for good luck.
I mostly pay table games and you could lay out a hundo that looks like it has been buried at sea for 20 years and they will take it as long as it's not counterfeit. I can just imagine a pit critter saying "Sorry, Sir, your money is old and wrinkled and we cannot accept it."
I only had an old $100 bill (1974 printing) questioned one time by a pit critter. He thought it was funny money until he took a closer look. He said he was surprised to see a bill that old still in circulation.