If it's possible, I would recommend getting the stereo mains first. If possible, I would look to get some at the start of the model's life span (for instance, if the speakers are newly upgraded from their previous model) so that you make it easier to buy a matching center in a year or two's time, you can be more certain of getting matching drivers.
I don't think it's as critical for the surround channels' drivers to match as it is for the drivers across the front (my rears are an old pair of NHTs, for instance, while I have B&W 600-series across the front).
So, I'd suggest getting a good pair of left and right mains, then adding a center channel, and then surrounds. If you already have a pair of speakers that can do your surround work, then you should be good to go.
I'm just never able to upgrade everything at the same time, so I look for ways to upgrade in ways that make sense... at least to me.