i would say you should definately charge if you do so. Your time is money!
If you don't do it & she has her caterer do it, they'll be sure to charge her. So if you decide to do it, you should to! I've never, personally, known of any cake decorators that slice & serve their cakes...it generally is something the caterers do, around here, at least. But there is a separate charge for it. I would charge an hourly rate for my time, if I were you, and then if you have to supply the plates, cutlery, etc. you need to charge for that too...unless the bride wants to supply those things herself or her caterer supplies them. Hope this helps... 
I don't know of a baker or decorator that slices and serves. Generally the bride arranges this with friends and family or pays the caterer to do it. If you do decide to do it, definately charge extra for it. I would even call some caterers to find out what they charge to do it and base my fee from there.
I have read that you should offer it as a service at a recommended fee of $25 per hour. Make sure the hours that you are to be there are indicated in writing so there is no question of how much you are to be paid in advance.
It is difficult to cut a wedding cake properly. There is a special technique in order to get the number of slices the bride and groom ordered. That can become a big problem if someone cuts the slices too large. I give my brides a copy of the instructions on cutting a wedding cake just to make sure. I feel it covers my commitment. They are so overwhelmed it goes right over their heads but if they come back after the wedding claiming the cake was not the size they ordered you can always ask them if they used the cutting diagram you gave them as a guide.
AlamoSweets
I've only stayed to serve cake when I've made it for a friend. It can be a slightly nerve wracking process, just be warned.
I would probably stay and serve for a very small fee, mainly because I'm nosey and love weddings and wouldn't mind it at all!
I was hired to cut a cake once, i didn't make the cake. I charged $10 a tier. I took my daughter along so she could cut the grooms cake at the same time. they supplied all the plates etc. We cut the cake, i boxed the top tier and when no one else was coming up to collect cake, i cut a few extra slices and plated them, then boxed the rest of the cake and cleaned up the area. they had the boxes from the decorator. We weren't there for much over an hour, and they said we could eat anything from the buffet we wanted, which we did not. they paid me $40. It was an easy job, apart from the line of people and kids wanting to take the chocolate dipped strawberries only.
This site never ceases to amaze me! Everyone is so helpful. sweetbaker, that was so good of you to look up those sites, and post them for her. I guess that is why I'm sooooo addicted to CC. Just a little pat on the back for all the helpful folks out there!
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