You've got it right. If your TV is mono (which I imagine it is since only one audio out) then connecting the cable directly to the TV will work fine.
The catch is that you will need to switch between DVD and cable TV by selecting "TV" or "Video" using the TV remote, and switching from "DVD" to whatever audio inputs on the receiver you have connected to audio out from the TV.
I'm not sure if the modern converter boxes have video outputs as well as RF connectors -- if they do then picking up a converter at some point is probably worth it for convenience. The main benefits are :
- sounds like your TV may be mono (not sure), imagine the convertor would be stereo
- depending on age of your TV it may not be able to tune all cable channels
- switching from TV to DVD would be one click on the receiver remote, no need to switch on the TV as well
Anyways, get it running by hooking up as you suggested then look at convertors at your leisure.