I do quite a bit of framing and have found that waviness can sometimes be a factor of the paper (too thin -- for instance Photo Rag 188) as well as the tape you use and the pressure of the backing board and technique.board.
Firstly, you should only use archival artist's tape along the entire top of the art work (photo). This is because it allows for expansion and contraction of the paper within the frame, based on conditions. (Personally, I've never liked mounting photos, maybe because I've screwed up too many, and in the process, trashed the print.) Once in a while, I've put small pieces of tape on the two lower corners, but you really aren't suppose to. The thing that greatly regulates the flatness is the backing. acid-free corrugated cardboard works, though I prefer archival foamboard. With the cardboard, you can also add a another piece of mat board. The key is stiffness, which with larger frames is harder to achieve. Finally, use the artist tape to seal the edges of the frame and backing to keep everything in place not just relying on the points. Also, use quality acrylic/plexi instead of glass -- actually has higher reduction of UV than standard glass.
Your waviness is a factor of solid contact and stiffness in your framing, not (usually) the paper. Harman is good heavyweight paper, so should be alright.
