Speigen cases, at least the Slim Armor one does NOT work with wireless charging as reported on xda-developers forum. If you want to charge wireless either buy the LG Slim Guard than replaces the back cover and has wireless and NFC pins but has like no lip (you should get a tempered glass screen protector anyway, so that case would then be fine anyway, the Quick Circle (the type that does have pins, though I'm sending mine back tomorrow. It's really nice if you're into flaps, I just realized that I hate flaps, especially that once folded around, which is does superbly though, the back of the phone no longer has that hand-in-glove fitting curvature).
I can't get the Slim Guard case right now without paying an arm and a leg and even then no store has it til August 29! Ordered a slide-on aluminum bumper and a very clear plastic case off ebay. Pure bumper won't interfere with wireless charging, obviously, and the clear one shouldn't, but if it does, I can pull it off in a jiffy. So there you go, either one that definitely works with Qi, or one that doesn't but can be pulled off in 5 seconds top.
Don't buy thick ones. Check out High on Android YouTube channel where he does a drop test of the G3 (and S5) with a Cruizerlite Bugdroid (TPU or similar) case on (concrete, head-level while walking, kid biking etc). Unless you're planning on playing baseball with the phone as the ball, you don't need some crazy thick case. Inertia - phone is super light, so impact on ground is much less severe than those heavy metallic phones. The thin metal rim of the G3 (real metal [oh wait, you know this already, but for anyone who doesn't yet own one - yet, I guess], not the horrid chrome-plated plastic that peels off in less than a year (Note & Note 2) seems to stiffen the whole phone a lot.
Whoops, long post, well, it's worth it if it keeps someone from crying over broken screen glass - btw "Once you go tempered glass you never go back". Nillkin seems popular, I hear and Speigen, maaah.