If you want the print to last, it should be against the glass since it will eventually become one with the glass. That is the purpose of the mat board or spacer.
In regards to the waves some of the stuff said so far is sort of correct. All paper based products, and in fact all wood is subject to waves and warping with temperature and humidity changes. The only way to truly stop a print from waving is the permanently mount it to the mounting board with something like dry mounting. However, if the mounting board is acidic, or becomes acidic it cant be replaced and it can hurt the print over time. This is more an issue when trying to preserve things for lifetimes. I have heard that most of Ansel Adams prints were drymounted.
So some waves are ok and natural, however, improper framing can make things worse. Yes paper will contact and expand with temperature changes and that is not the big problem. The problem is if you completely tape one or more edges of the paper down to a the mounting board so the paper is not free to move. The problem is that the paper and the mounting board will not expand and contract at the same rate. If the mounting board shinks faster than the paper the paper is forced to buckle. That is why the paper needs to be free to move. Proper archival mounting usually means just hinging perhaps two points at the top so the paper is mostly hanging inside in the frame and the only fixed point is the just a small line between the two hinges. Using a mat board that overlaps the image can help keep the edges of the paper down without restricting the paper from expanding and contracting under the mat. Photo corners, mounting stips, or mat cut outs are other ways to mount the image loosely without adhesive.
You can see an example of hinging here:https://www.framedestination.com/pictur ... -mounting/
When you look for a framer, try to find on that is Certified by the http://www.ppfa.com/
It will not be a guarantee, but will indicate you are much more likely dealing with more of a professional.