I used to wander all over Scotland and never booked anything, though always just off season.
I'm older, lazier and better off these days so I tend to book accommodation.
Tourism in Scotland does appear to be going through something of a boom in recent years. The internet age seems to have given it a boost, and more recently the weak pound.
There’s been a big boost in tourism in the far north as a result of the NC500. Plus Skye seems to be bonkers from April through to the end of October these days.
You should be OK in May, bar Skye, but then I’m not quite sure how big the NC500 effect really has been.