I often find myself in situations where I have to choose between a high scoring word where the opponent can stick s to score highly, or if I should pick a lower-scoring word that you can't add "s" at the end of to make a parallel word. I suppose it's different for every board, like depending on tile position and lead, but as a general rule, what cases, if any, should I play around "s"?
This is a balance between how many points you are scoring now and how many points you can potentially give up. If you play the lower-scoring word, how many points are you losing? Do remember that opponent might not be able to take advantage either, if they don't have an S.
Usually for any square except a triple word score (TWS) tiles, it's not worth giving up many points in order to avoid the potential hook. With TWSes you need a bit more math. How many outstanding tiles are there that can hook? For example if your word is ITCH, then there are exactly four tiles that hook to the end: the two Y's and the two Blanks. If all four tiles have already been used then you should have no qualms about using it. On the other hand there are significantly more tiles that hook to the front of ITCH. Therefore the potential front hook is much more dangerous than the back hook.
Because whether it's worth it to play around an S depends strongly on how many tiles have been used, what the score is (if you're behind you must take chances), etc, it's not possible to give more precise guidelines.
One more tip: if both blanks and three S's have already been used, and you're holding the last S, you should absolutely play the word without the S and keep the hook for a later play.