I used to be a FEAD engineer. (front end accessory drive)
Running w/o a tensioner and a standard off the shelf belt is not a good idea. In order to make it work you'll be having a belt that may stretch and relax at hot temps causing slipping or one that's too tight and will put excessive load on the accessories..wearing their bearings out prematurely.
When the AC compressor starts, the clutch doesn't slip much at all. This causes the highest load on the FEAD and is what the OEM tensioner is speced for. If you have removed the AC you can generally run a lower tension by drilling a new hole in the mounting bracket and/or running a smaller pulley.
Its not a big power consumer but a little TQ can be freed up by reducing the belt tension.
Here is a picture of my set up. Since this pic was taken I've put on underdrives a new smaller idler pulley. The stop tang is now riding in the middle of the swing and the belt spacing is about the same.