It's not going to hurt them to compress them when you tie them down, so long as you don't leave them like that for a long period of time.
Say, if you are going across the country, and it's going to take you a few days to get there, I'd probably loosen them up while I was parked for the night..
If you are just taking them out to the ORV park, and only going to have them in the truck for a couple of hours, no problem..
If it worries you though, you can always buy a "fork saver", they're about 40 bucks, and go between the tire and fender, and don't allow the forks to compress more than about an inch.
If you are handy, you can make one. Figure out the distance between the tire and the fender with the fork at the top of the travel, cut a 2x4 that length, and you're done.