Can I ask, is there any disadvantage to waiting to firm or insure.
If the potential firm choice opens applications for accommodation early, restricts it to firm offer holders but runs the system as booking a specific room in real time, then there is the risk of missing out on getting one's preferred accommodation. However, some / many universities run the system as inviting applicants to express preferences from a certain date, but rooms are not allocated and paid for until after results day, when offers are confirmed (or not). In that case, there is certainly no need to rush into making a decision that one might regret.
Offers are only rescinded if the offer holder fails to confirm their firm and insurance by the relevant deadline - it's not a first come, first served set up.