Hi, thanks for posting!
The straight answer is Yes. If you have a core that isn't assigned to VS or to NCrunch, then neither the VS UI nor NCrunch can touch this core. Note that from VS2012, VS does quite a bit using background threads, so it may still intrude upon this core a little (NCrunch cannot prevent this), but if you're looking to try to reserve some CPU for other tasks, then keeping a core unassigned may very well help.
I do also recommend keeping a close eye on your system resources to be certain the CPU consumption is the cause of your system responsiveness issues. Sometimes there can also be other causes (i.e. excessive I/O etc).