We don't, either.. but like others, discuss with them from the first meeting what they want for formals... including pairings, match ups, and so on....
We often find out then about interesting things... the bride would rather her mother not even be there, but will pose with her in formals - what she really wants is a formal with her grandmother... the groom's parents are divorcing, but have promised to behave.... fun stuff.
At the same time, I give them an idea of how much time it will take.. and we decide when they will be done (before or during reception)...
I have seen some wedding photographers that insist on doing the formals BEFORE the ceremony.

I can't imagine that for some reason.
Oh, and on occassion, you'll have the Wedding Nazi such as the church appointed wedding manager for an upcoming wedding... she said "We will NOT be doing formals before the reception. We will not keep the guests waiting!" The bride rolled her eyes and looked ready to bolt for the door.. so we came up with a compromise that will have us pulling people out of reception as needed. That will be fun... not.