I have several cross stitch pieces that are matted and framed. All of them have glass. My pieces have been framed in this manner for over 6 years and there has never been any condensation. My sister has pieces framed in this manner for at least 15 years with no condensation.
If you have taken the time to complete a nice stitched piece then you would want to protect it while showing it off. Glass will keep your piece from getting dusty, dirty and protect it from any further damage from the oils in our hands or household smells. As indicated by Caerleon, pieces should be matted so that the glass never touches the stitches. I usually double mat my pieces, but one would do the job just fine. Also most pieces are mounted onto a foam board and then popped into the frame snuggly and then covered by brown paper on the backside. I believe all of these materials to be "breathable" and help prevent condensation.
If you wash your piece before framing it then make sure that you only hand wash in very mild soap without rubbing. All stitched pieces should be blocked and air dried flat. Make certain that it is absolutely bone dry before framing, if it is the slightest bit damp it will form condensation. And if you accidentally get your piece wet while it is in the frame (i.e. cleaning the glass) then you should try to remove it from the frame carefully so that it can dry and then place back in frame. To avoid getting your piece wet once it is in the frame NEVER spray cleaner directly on the glass. Glass cleaner should always be lightly sprayed on a cleaning cloth and then applied to the glass by wiping with the cloth.