I personally wouldn't want to get on her while she's growing rapidly, but by all means, mouth her, get her responsive on the ground, chuck rugs/saddles etc. on her, boots on her etc. Do everything with her other than actually getting on a riding, and also avoid any serious lunging at this age. At 2 years old they tend to like getting into mischief and need something to keep them occupied, doing a lot of work on the ground, mouthing etc. will give her something to think about, without affecting her growth :)
It's certainly a common practice to get on them for a week or so then put them out until they're 3. But it really does depend on the horse, if it's a horse that is looking quite mature at 2 - 2 1/2 and has a good sound mind, then sure if you are only going to sit on them and teach very basic aids. I'm not a huge fan of this practice and think it's more appropriate to be teaching them as much as you can from the ground before worrying about getting on board.
You could always - if you're brave/a very good rider/mare is QUIET - slide onto her bareback. Or even just lie across her, sit on a fence next to her so she gets used to you being above her, pony her off another horse again so she gets used to you being higher than her etc. There are so many things you can do without having to back her :)