Let her stand, it's easier to wash her that way, just watch her and help her if she's going to fall. It's great for them it helps their legs, so just let her but again watch her to make sure she doesn't fall. My son is 2 and sometimes he falls, you just have to be there to catch them. Make sure you don't put too much water so she has room to stand and still play in the water.
when my son started prefering to stand I felt uncomfortable since he wasnt very steady yet, but he just didnt want to sit! I got some non-slip bathtub appliques, and some new bath toys which made bathtime a little easier. Plus when he stands I can wash him more thoroughly. You just gotta keep a hold of her so she doesnt fall! and if you fill the tub with bubble bath she might sit and play! my son loves to sit now (he's 14 months old) and splash away, but to get froom one end to the other he stands and walks. *aaargh* that makes me nervous lol
I considered getting a bath seat for my son since he tries to stand too but like the others said I've found it much easier to wash him when he's standing. Plus he would probably try to climb out of a bath seat anyway. It does make bathtime a little more work trying to make sure he doesn't slip and fall but as long as you're careful with her than she will be okay without a bath seat.