Well, usually the horses that are going to react badly to spurs are the overly senstive types or ones that have been abused by them before.
Since we're talking about a lazyness problem, I doubt she's "overly sensitive" and you know there's no history of abuse with them.
Pretty much, if you can stand on the ground and ask her to move her hindquarters over and use the handle of a hoof pick or a finger to nudge like you would with a spur, and she doesn't jump/kick, you'll probably be fine in the saddle. It's really just a back up aid, and she'll have warning before you use it (calf pressure progressing down to the heel). Just remember to NOT accidentally spur her by clamping down with your heels if she does jump forward or move out quickly (the first few nudges generally get the strongest reaction because it's new).
With proper use, I doubt you'll see any problems.