If you are getting a new wheel built, just get one with a fixed/free (flip-flop) hub and put them both on. That's what I did bake in October, and haven't ridden that bike in freewheel mode since then. Perhaps it's a bit of a waste to have a cog on there that I never use, but it's nice to know I have the flexibility, and it's much quicker to change out than taking a FG cog off and putting a freewheel cog on.