A: I started singing after the fact. I actually began by singing chorus in my band, they made me do it. I think you have a better chance at getting a gig if you can sing as well as play. Things I've learned about singing from singing myself and watching students at college:
Pitch - Singers who can play an instrument have good pitch. There are things you can do to change your singing voice to some extent but, I don't know, some singers just have great tone. Pitch more so than tone can certainly be developed and having a good relationship with an instrument is sort of a short cut. So if you play guitar already, you'll have a decent shot at singing as well.