When rendering a Maya scene using a batch script, I always set the number of CPU threads to the maximum the processor can handle simultaneously. I often free up a core or two in order to work on something else, but all without ever stopping the rendering process to edit the source batch script. I'll cover how to do this in Windows NT 6.1 (Windows 7).
Open the Task Manager. (The keyboard shortcut for it is Ctrl+Shift+Esc)
Go to the "Processes" tab. Right click the mayabatch.exe process and choose "Set Affinity"
Now just deselect any cores you want to free up, then choose "OK". Do this at any time you need to dynamically and instantly alter the amount of CPU threading any user process is assigned.
I use this for GUI rendering in a video editing program, command line media transcoding with x264.exe and ffmpeg.exe, GUI and batch rendering in Maya; basically, any time to get some simple control over any multi-threaded task without having to interrupt it.