No right or wrong way to go about things- it's not "better" to record guitar and vocals separately- it *may* be more appropriate in certain situations, depending on the recording setup, the singer, the song and what you plan to do afterwards.

If the end result is going to be a guitar/vocal performance, and that's how you always rehearsed and performed it, then your best bet is, as Matt says, to get enough takes that you can replace any mistakes, which means you perform it the way you're comfortable.

Soundwise you do limit your options for later on, but if you're planning a relatively straightforward, natural-sounding recording, there's little point in having options you're not going to use if it compromises the performance. In practice, as long as the guitar and vocal are both going to be present in the mix, you don't need total separation, just enough to be able to balance them.