Depends on where your crossovers are for your subs. AFAIK,there really is no scientific debate about localization of deeper bass notes (the typical 80 Hz crossover works for most folks, but it varies somewhat byindividual). And my experimental/experiential "research" is entirely consistent. When I first got a sub for my car, the crossover was too high, and it sounded like the bass player and part of the drummer were in my trunk. when I switched to a 70 Hz crossover, the whole bass player sounded like he or she was in front - - maybe right or left, but not coming from behind me. Although the deeper part of the notes actually came from behind me, it did not sound that way anymore. Same thing with my current HT setup. Sub is slightly left of center, and bass players on the far right sound cleanly and clearly on the far right.
Because bass notes from real world instruments include overtones that reach well into the localizable area. When I cross over my sub at 60 or 80 Hz, it does nto send every bass note with the fundamental tone 40 Hz (about an E) to the sub. It sends the part of the sound of that note below say 100 or 120 Hz (accounting for rolloff and assuming an 80 Hz crossover) to the sub. The rest stays in whatever main speakers it started in. Of course one can localize a bass instrument or sound when the localizable frequencies are in a specific location.
But if you find yourself turning to look at your subwoofer when the bassist goes low, or on each kick drum beat, your crossover is too high. If you don't, that tends to prove that the lower part of the bass is not localizable.
I would suggest that, technically, if deeper sounds from the surround speakers (assuming they go to a sub now) don't sound disjointed or jump to the location of the sub, you will not experience improved localization with subs on each channel. But if you're crossing over failry high, or even if you just want to do it for fun, it's not likely to hurt anything to add subs. Although since most program material puts the deeper tones across the front and in the LFE, some would argue you're not getting the best use of the subsif you feed them only the individual surround channel. Again, though, your system, and you get to make yourself happy.