I add hair to my braided hair b/c I don't like using nets (although it is a personal preference). I like getting to my scalp. The extra hair is a bit of security (It's thin, I don't use a lot) I like protecteing my ends with synthetic hair.
In addition to what everyone else mentioned would also ask her the following:
What braid pattern she plans to use in your hair. Straight-backs tend to lay flatter, zig zag puts more pressure on your sides, circular tends to not last as long as others (It's one long braid), so on and so forth....
I would have her stop immediately if any of the following occur (You should address these prior to braiding):
1. Braids too tight
2. Braids are too big
3. Doesn't leave your edges out
4. Braids while hair is wet
5. Doesn't add any "juices n' berries" for slip
6. Snatches the comb through your head
As a braider myself, some of these may happen. Afterall, she can't feel what you feel, it's your head. Just politely ask her to stop and request that she braids a looser, smaller, add oil, whatever it maybe....
I would also suggest,
previewing your hair after she completes a couple rows. You can get an idea of the size, tightness, and overall feel of her work. At this point it's easier for her to change what she's doing per your recommendations. Too many people let the weave-ologist finish without ever evaluating midway through the process. NOT A GOOD IDEA! AFter 2-3 hours, everyone is exhausted and you're more prone to just let things "ride."
Do not be afraid to (politely) set your expectations. The installation is critical. If she is not open to your suggestions or become defensive/rude. STOP!!!!! Again, I say STOP!!!! You don't want a p-ssed off braider all in your head.
At this point grab yo' purse, throw on a half-wig and leave!
If all goes well, then we need to discuss the takedown process. Installation/takedown have the greatest potential for you to lose ALOT of hair!