I recommend you do everything you can as far as washing, conditioning and moisturizing your hair before leaving home. Basically everything that needs to be done except for the braiding, do at home. I have two bad experiences that I remember with getting my hair braided. One was the braider using that awful Pink Oil on my hair...next time I'll take my own moisturizer, just in case the braider says it doesn't have enough on it. I used moisturizer last time sparingly because I didn't want the hair to be too slippery.
The second one is, I was getting my hair braided constantly without relaxing in between. The braider told me she needed to blowdry my hair to straighten the new growth some. While she was blowdrying my hair, she was yanking it and my hair was like straw and broken when she was finished. She also didn't use any type of heat protectant. Next time, I will blowdry my own hair at home...gently and protected.
My experience with braiders is that braiding is what they do best...everything else they're not really good at.
I'm glad your cornrows turned out well, though! What style did you get? How long are they? I'm getting mine cornrowed next month!