If you happen to have TOS lying around, do watch it first. I think you will enjoy TOS more on its own merits than compared to TNG, and there will be the occasional reference to TOS in TNG that was meant to be fun for fans who had seen it that will just blend into other comments if you haven't seen it. (It isn't like a joke that's only funny to some, and therefore unfunny to others, it's like noticing that a character in your favorite tv show has a coffee mug that references your favorite movie: it tickles you if you get it, and if you don't it is just a mug.)
But don't put off watching TNG to wait for TOS. That would be like not eating the cheesecake because you don't have any strawberries. (Actually, bad metaphor, as strawberries improve cheesecake far more than TOS will improve TNG.)