I, personally, believe that nearly all private parties should make use of a DJ instead of a cover band. Others may disagree and that's fine, they can hire a cover band. Simply put, with a DJ you get the original songs sung by the original artists and you don't have to worry about them "knowing" a particular song.
It's instructive to consider the overall role of the music maestro in your reception. Although many people don't realize it, there is a lot more to running the music than simply playing tunes. A good DJ is trained in getting a response from the crowd and getting them up and moving.
Let's face it; the majority of people aren't comfortable getting out and dancing. In line with human nature, there tends to be a domino principle at work in getting a wedding party dancing that is paradoxical in nature. An empty dance floor is death because no one wants to be first. The more people that get out there dancing, the more people will feel comfortable getting out there to dance.
Roughly 10% of your wedding party is going to be out on the dance floor no matter what. Another 10% won't get on the dance if they have a gun pointed at their face. The remaining 80%, however, sit on the fence. Most of them want to dance, but they are too self-conscious. This is where the DJ comes in.
During your interview process, you need to find out specifically what types of things the DJ does to get the crowd involved. The better DJ companies will be able to show you video footage from other events, demonstrating the methods used to get the majority of that 80% out on the dance floor, having a good time. Standing behind the booth shouting encouragement just doesn't cut it.
Because I don't want to bias you against smaller DJ operations who cannot afford to put together promotional video, your DJ should be able to tell you (in detail) what types of things are going to happen to encourage people to loosen up.
As with all things in life, you get what you pay for. The guys who will DJ a four-hour wedding reception for $100.00 probably won't be as effective as the guy who changes $650.00. Part of the increased cost ought to go to giveaways (hats seems to be giveaway of choice because it's almost as if putting on a silly hat unleashes the alter ego in a reserved person).
If the DJ you're talking to seems clueless about crowd control then don't give them another thought. You need a professional who will take charge and manipulate people into dancing knowing, full and well, that once they start they will enjoy the reception that much more.