I don't think it makes any difference in terms of CPU load. The CPU is going to compress frames at the end of the process, so whether you start out and 1080 and then scale or start at 720 and don't probably won't make a lot of difference.
It may make a difference to your GPU, that has to render the full frame before it gets scaled and compressed. But if you're using an external capture device and not overloading your PC's GPU while streaming, that probably doesn't matter either.
Do a test, look at the log. Open that Status window and check for GPU or CPU overload messages.