I wouldn't give her any juice unless she actually 'needs' it. My daughter has suffered from chronic constipation (she is 16 months now) and we started giving her a teeny bit of juice when she was 5 weeks old for that. However, it was 1/2 oz juice with 1/2 oz water once a day and only when she needed it. I really hated doing that and as soon as we found a medication that worked for her, she didn't get juice again until after 6 months.
I think it is really what make you feel comfortable. My daughter did fine with the juice, but I didn't like it. It made her a little gassier than usual, and therefore fussier, but that could have been the constipation, too. I didn't like doing it, so I quit as soon as possible. Also, if her digestions is doing well, adding juice could really throw it off, give her unnecessary gas, and possible give her diarrhea.
Personally, I would not give her any, but if you feel like you need to, then do it. You will know what is right for her. Trust your mommy instincts! ^_^
Good luck!