It has never been clear to me what the difference between project profiles and render profiles is.
There is a page in the manual http://userbase.kde.org/Kdenlive/Manual/Projects_and_Files/Project_Settings that says
" You should carefully choose your project format and select the one that fits better with what you want to output. All video operations on the project (like compositing, scaling, etc) will then use this profile.
For example, if your goal is to create a DVD, you should use a DVD profile with the correct frame rate (PAL / NTSC) and display ratio (widescreen or not). "
But why should we do this? Surely it is the rendering that defines what sort of video we get.
I guess the manual page goes some way to explain what the project profile provides. Is it saying that if you don't match the project profile to the profile you render with at the end then the video operations that Kdenlive does will not be accurate because the frames per second or line count will not be correct ?
Can we be more explicit about what the project profile does?

Thanks al25fps and moorsey. My take home from what you are saying is that the manual I referred to is wrong when it says make the project profile match what you want to "output". It should in fact match the format of the files you are going to input.
I guess this means we have a problem if we are combining sources from disparate formats. Would it be recommended to convert disparate formats into a common format before doing serious editing on them?