I tried the fair thing last night, and it does work decently well. Like someone said before (forget who), you just have to make sure to retreat it and/or mix up your landings so that Wario has less of a chance of grabbing you when you hit the ground.
Keep an eye out for opportunities to grab. The fthrow/dthrow > usmash comboes help out a LOT with getting mid-percentage damage. Don't force it, though, or you'll get all kinds of punished.
Ugh, I really hate this matchup, though. It's not particularly hard (I'd still keep it at 45:55 Wario), but it's so ****ing campy and nearly always goes close to time. Pretty much a bunch of jumping around, attempted baiting, and hit & run from both characters.
At high percentages, it's pretty easy to keep from getting killed. The hardest move to avoid is the clap, imo: it has a decently large hitbox, goes through all of our aerials (or trades in his favor), and he can bring out the hitbox close to the ground. His fsmash is annoying, but with practice, you learn to see when it's coming..... be careful about trying to punish its endlag, though, because if you're even the tiniest bit late, he'll throw out another and beat you with its super armor. If you see him winding up, shield; in my experience, most Warios won't attempt ftilt kills much, but it's still something to watch out for. Fthrow/bthrow (I think it's both..... whichever ones he whips you around for) kill near the edge on stages with walk-offs, but on regular stages, DI up and you should be fine.
I seem to kill the most with fsmash and edgeguarding in this MU. Fsmash seems counterintuitive since Wario's in the air most of the time, but walk away > reverse fsmash can mess with his spacing as he tries to land/hit you. When he's off the stage at stage level or below, he's really predictable...... his up b sucks, so he's gonna use his bike, then hop off and usually go into an aerial. Just smack him off his bike. (I usually dair, bair, or nair.)
Usmash attempts always seem to get me killed in this MU. @.@ It's too laggy on block, and good Warios will avoid committing to anything we can punish within usmash/hyphen smash range. EDIT: Fsmash is laggy on block as well, but usmash has less shieldpush and lags you within range for fsmash.
I land a lot of utilts, but they rarely string into thunders because of DI on Wario's part. Meh.
Just my 2 cents on the MU from my limited experience.