I would say when the regrowth starts to look too stark against the current colour. I haven't yet coloured my hair, but here are a few examples in my circle:
One of my aunts - fair skin, blue eyes - but she was dyeing her hair a fairly dark/auburn shade, and it was starting too look too harsh, and the grey growth coming in was exceptionally obvious. Too many touch-ups etc. My Mom convinced her to go blonde instead of the dark brown/burgundy-ish auburn colour she was using. It looks *great* and you can hardly see when more grey comes in. She doesn't need the touch-ups as often and it's a good colour for her.
Also - my Mom. Natural blonde. Went white over the years - for awhile, this was kind of neat - it was looking more and more platinum. But she's even more fair skinned than my aunt, and it was all starting too look washed out somehow. I convinced her to go darker blonde - this also worked beautifully. More definition to her face/colouring, but not 'starkly' coloured either. She doesn't need too many touch-ups etc. The grey in her case (more white really) seems to almost blend in with the blonde hair colour.
My other aunt - she's still doing the 'too dark' almost burgundy-ish thing. I think she'd look better going a bit lighter or letting her grey grow in - she's naturally a brunette. Fair skin, dark brown eyes, her natural colour (before grey) was always a dark brown. I think if she's going to keep colouring it, she should go much lighter, imo. Or - I also think just letting the grey happen would look amazing on her. She's very pretty and I think the grey she would get would be very striking on her.
Meanwhile - my best friend has naturally dark brown hair. Guess what? There's a chunk of grey in it, and it looks so neat! I love it on her. She's never coloured her hair either, and I think that streak of grey looks super. The same thing happened with her Mom - when she started "going grey" - it was in these fabulous streaks/chunks. Best friend's Mom is now all-over grey, but the colour is really nice. Naturally dark brown hair, fair skin, brown eyes again.
So, I guess it depends on your colouring - how much hassle it is to keep up the colour and so on.
I can't tell if I have grey or not. If I do, it isn't showing up against my blonde hair in any discernable way (yet!). At some point, when it does - I'll probably do just what MaryK and my Mom did - colour it blonde.