No. Stereo imaging is not impaired by a sub, provided that you've managed the x-over properly. Even if you screw that up, you're probably okay on the stereo separation front, so long as you're crossing resonably well below 100hz.
Marty
PS As Jax points out, a lot of things go easier with subs when there's more than one to work with.
Marty
PS As Jax points out, a lot of things go easier with subs when there's more than one to work with.