Seconding (or thirding) all the responses about The Valiant being pretty self contained and spoiler free. I think the worst you will encounter is your own realization as to which characters are there and which ones aren't. Not necessarily spoilers, but you may wonder why (putting this in spoilers so avoid if you want to be really clueless about what goes on before)...
spoilers:end of spoilers
Shadowman, Peter Stanchek, Kris Hathaway, and Flamingo (off the top of my head) aren't there. Or perhaps why GIN-GR is there.