It's impossible to give a blanket statement about whether a sub will help your specific setup, but if you're using a stereo pair of FRFR speakers in general, then yeah there's a good chance a sub will help, as most stereo FRFR speakers don't really extend evenly down to the lows except for the highest-end stuff, so it's likely you won't get accurate representation of the lows, and a sub will help.
Another thing is that if you don't use a sub, you could get used to dialing in WAAAY too much bass in your presets without knowing what's happening below what your speakers are capable of outputting. I did that with a pair of monitors for a while, then once I got a sub I realized how out of control my lows were and corrected them in a hurry, which helped my sound a lot.
And finally, if you're using a sub to dial in guitar tones, and if you're on this forum then that's likely... you're going to want a 9 or 10 inch sub at most. Anything bigger has a good chance of loosening up your low end too much.