Like MexicoMike, I use a vocal harmonizer, TC Helicon Gtx; so I practice plugged in. The harmonizer doesn't correct your pitch, so if you are off, so are the harmonies. As Mike experienced, you have to understand how the harmonizer gets those notes. One thing I have found while using this: I don't "slide" to a vocal note, because the harmonizer will do that as well... it can sound a bit demonic. Same thing with bending notes on the guitar. You can dial in the amount of effect (mainly reverb) and harmony; the Gtx sounds best (from my perspective) with the harmony set in approximately the 9:00 position, so that the harmony is not as loud as the melody vocal.
I run the guitar through the Gtx and out to an amp so I can make adjustments to the guitar without affecting vocals. It still uses the guitar to determine how it will output the harmony. If you are in a situation where you are running direct to a house PA, you can leave the amp at home if you want and use the Gtx to blend the amount of guitar and vocal to the PA, giving you some adjustment in sound. Or, leave the amp in the chain, run the vocal out from the Gtx to the second channel on the amp and then run a direct line out from the amp - this allows you some control of the tone of both the guitar and vocal, AND lets you use your amp as a monitor (I use a Fishman Loudbox Mini).
People are fascinated by the harmonizer, and will often get closer to see if someone else is signing with me.
The harmonies add a nice variety... try playing some Beatles, Eagles, or Everly Brothers songs with and without the harmony - the addition of the harmonies of your own vocals is a big plus.
My "back up singers" are able to keep up with any changes I make on the fly.
Best wishes,
Captain Jim