Unfortunately for Rockets, they are really going through some growing pains and haven't been able to keep up for the demand of their products. Ever since their sales in the Fall of 2007 and their highly regard MFW 15 Subs got ranked pretty high, everyone pretty much flooded their gates. The Rocket center weighs 50 lbs and is 2ft long, 1 ft high, and 10.5 inches deep. I didn't realize why they nickname it "Bigfoot" until I opened the box it was in.
The center is very important to a Home Theater. During a movie, the "talking/voices" will be coming out of the center speaker. Your two main (L,R) speakers can do the work without the center but will you be satisfied enough with it? Will you be happy setting the speaker on the side? These two questions, only you really can answer these questions, depending on how picky you are. Some people are extremely picky, everything have to be aligned, centered, perfect height or it will drive them mad....fortunately, I am not one of them. For example, my side speakers needed risers to match my fronts (in heights) but I didn't buy the risers because it was $50. My center speaker is not in a perfect height and not quite centered because my audio rack is too high and would interfere with my video projection (I have a projector) and so I have my center speaker on top of a small coffea table, next to the audio rack (it looks ghetto). For me, I rather have a misplace center than a "no" center but that's just me. But again, I don't know how "displaced" it will be at your place to the point it looks weird. Will the sound be displaced enough for you to be unhappy? I don't know. It doesn't bother me, but it doesn't mean, it won't bother you. I hope someone else can put in their 2 cents. Always good to have more than one opinion.
Best solution is to find a speaker that will fit your place. The good thing about speaker is that there are a lot of choices you can go with. It's not the end of the world if one doesn't fit your needs. What if you find one that will fit the mantle but its not the wood cabinet finished you want? Will you compromise?
Take a look at these as well: PSB, NHT, Monitor Audio