I bought leveling blocks day 1 I had my RV, needed them at my very first site. I think there's a lot of gimmicky RV gadgets that aren't really necessary for enjoying the lifestyle, but having something to make the RV level I'd say is crucial at least for me - the fridge is the one thing I can't afford to replace in my RV, if it breaks I'll literally need to cut a hole in the fiberglass to get it out, the door an windows weren't made big enough. If you're too cheap for the commercial plastic blocks, you can also cut wood down to size I've heard, works just as well.
Been a full-timer for over a year now and I'd say I've used them at about 80% of the sites I've parked in. I have a bubble leveler on the front of my trailer and I'm good enough at reading it now to tell at a glance how many blocks I'll need. It can get annoying trying to level it in the rain, I try not to travel on rainy days.
Good luck!