I didn't find any info on this, so I decided to test it on my own.
I ran mksquashfs with just 1 thread (-processors 1) with HT truned on and off.
It turned out that, with HT enabled, despite a single thread used (or 1/4th of the no. of threads the processor had to offer), all of the processing power of a single core was utilized. With HT turned on, the task finished in 46 seconds and with HT turned off, it took 45 seconds.
So I suggest HT turned on always, even for single threaded applications.