the textbook answer to this one is that you would want to match your center to your mains. it's unlikely the Polk has the same timbre (or "voice") as the the Wharfedales. you might see (errr, hear) a skip or interruption in the sound as it moves from the left speaker through the center and on to the right. in theory, you should find the Wharfedale center that's matched to your mains.
you can adjust for small mismatches by moving the center forward or back a little off the arc of the left-center-right speakers, but it won't help much.
all that being said, if you can get the Polk from a decent company that will let you return it for only minor fees, it might be worth a shot.
this also applies if you use your system mainly for H/T. I haven't seen anything written about timbre mismatch for SACD or DVD-audio, and I don;t have any experience with that part myself.
