We asked the venue what time they would recommend based on our ceremony time. For a 1pm ceremony they said anytime after 7 and gave us approximate times for the day based on their experience
Our ceremony is at 2pm and we're asking our evening guests from 7pm onwards, but we're putting on our evening invites something along the lines of:
The request the pleasure and venue details then:
From 7pm onwards, cutting of the cake to be held at 7:30pm.
This lets people know that they can turn up from 7pm onwards, but if they want to see the cake cutting they need to be there by 7:30pm. Some of our friends are notoriously late and we thought that way nobody could complain if they missed the cake cutting!