Before plastic inserts the Independent garage I used put it on everything that was exposed (within reason) very few of the old cars I had developed as much rust as similar models he swore by the stuff.
Any re-sprays he did automatically got coated and if it wasn't on he made sure it was after he serviced them
Dad who also uses the same garage has had cars which should have ended up on the scrap heap for years more than other people again through liberal amouts of waxoyl (Austin's, Morris, Vauxhall, Renault, Ford the usual suspects). Only plastic inserts have really replaced it's use for wings, valences and sills
If you don't have plastic protection it's a good investment and always check it's state once a year (usually before winter !)