Yes. Part of the charge for the extra person involves the whole use of another bed. Figure out how much work it is and charge accordingly. If you have to haul the bed out everytime, clean it up, get the linens out, etc, it's worth something.
We have a $30 extra person charge, but no rollaways. One suite has a day bed and our cottage has a sleeper sofa. With any additional guests you are doing more work, more laundry, more food, utilities, etc., so an additional charge is very fair.
We charge $25 for a third person in a room, with a roll-a-way. But we don't charge for the roll-a-way if there are only 2 people and they just want the spare bed...we probably should but so far we don't.
I used to not charge and no one batted an eye either way, an addtl $25 or $30 is $25 or $30! So i now charge, every time. One of those things I have grown into...hotels charger per person, that is a fact, a single vs a double and a third person.
we have 2 cots which we do for free as they are dead easy to put up and they have to provide their own linen to be honest we only use them about twice a year. We have one fold up bed and charge $30 for it but the last time it was used (it is heavy and a pain) we set it up for this woman who was staying with her young son and her other son was supposed to be staying one night as well but he decided to not bother and stay with his mates instead was really angry as had to move furniture round and everything and felt bad charging her as he hadn't come but we did all the work and that is what you are paying for.