Supplementing with sheet cakes is fine and all, but remember that they're not going to be pretty, and they may just be plain white with a border. Because of that fact, you probably don't want it to be out with your other cake, let alone in the center of our cake table. Let the pretty one stand out because that is, after all, why you got the nicer one.
For example, here's a beautiful white wedding cake
|Via Lulu Cakes|
That being said, don't put it next to this
I'm sure the sheet cake tastes fine, and that border is piped on pretty well, but it's not centerpiece material, and definitely shouldn't be on the cake table at a wedding reception.
|Plain sheet cake. Yay.|
Even if the sheet cake has roses, writing, or pops on color on it, it should still stay in the kitchen. You bought the elegant tiered cake for a reason. If you MUST have your sheet cake out to present to everyone, put it behind the main cake, put the main cake on a cake stand, pedestal, or plateau, or if you have a dessert table, have the sheet cake all cut up and ready to go (if the kitchen staff isn't passing out pieces of cake) so then your guests can grab it and go with no problems.
Another problem with having both cakes on the table at the same time (if not purchased from the same place) is the fact that most of your guests will assume both of the cakes came from the same place, and most decorators don't want that. Not because they're snobs or anything, but because a tiered wedding cake is a specialty item custom made for your special day. Sheet cakes are more often associated with grocery store bakeries, or local high-volume bakeries. Those types of bakeries aren't inferior or anything, but they're just different beasts, and cake decorators want to keep that distinction.
The best thing to do it keep the sheets in the kitchen. If not, try to highlight the main cake as much as possible. Remember: you ordered the special cake for a reason!