There's a plugin by a guy called Aliex Hilton - the A1 Audio A1StereoControl. It has a "Safe Bass" feature, so you can say "Anything under XXX Hz must be in mono", and a "Make everything mono" button, and a "stereo width" control. Drop it on your master out and you can mono your mix very easily to check that what you're doing is stereo compatible.
Some plugins are stereo compatible, some aren't. I think (not 100% sure) Massive's Dimension Expander (and Xfer Recordings Dimension Expander) are designed to add stereo width, but be mono compatible. Meanwhile, be careful of doubling plugins that use the Haas effect to make a mono sound stereo (sending delayed signals to the extreme left and right) because if you collapse the results to mono, you might have accidentally comb filtered your music. Waves Shuffler... I think that works by panning different frequency bands left & right, and I think that's mono compatible.
Basically, if I'm artificially widening a sound, I try to remember to use A1StereoControl to check my mix after I've added the effect, to make sure I haven't messed it up significantly.