if you've only got surface rust, I'd go to the trouble of restoring the metal rather than spraying in the liner. I'm not implying that everyone that sprays a liner is hiding dents - in fact some of them look really nice. It's just my personal opinion that I like new trucks with spray in liners, I like old trucks (i.e. like ours) to show the metal - even if it's not cherry.
I guess ya gotta ask yourself - are you spraying because you won't have to worry about dinging it up in the future and it's easier - or because it is what you actually prefer to have in there? If the latter, then spray away. One more consideration - resale: a spray in liner seems to detract from the resale of our old trucks, at least that's what I've seen. May not hurt the price you sell it for, but it might take longer to sell it because many folks immediately turn off looking at a bedlinered truck.
My 2 cents only