The best time to do it was in 2007. Well, in fact she was a redhead until the start of the tour and she looked great, don't know why she decided to go blonde again. So if she decided against the idea when they first reformed and recreated their 90s look onstage I really doubt she will go ginger this time.
I don’t think it matters, if she wants to then go for it, if she doesn’t then leave it. I never got why she changed her hair colour from the reddish colour she had literally just before their reunited in 2007 though?