In a rotary there is simply no reason to ever go inadvertently IMC, assuming VFR pilot and machine. Just go lower and slower, and when you get too low turn round and go back, or land. But what ever you do, never, ever, lose visual contact with the ground. And if you need to turn, do so to your side, (ie to the right in most helos) so you can keep the ground in your door window as you turn. End of story.
Victor two: you could have descended and slowed, and then turned back in VMC if it was deteriorating too much. No doubt because of their aircraft's inherent stability, and the fact they tend to fly higher, fixed wing PPLs tend to go IMC more often, but such an approach in a rotary is a recipe for disaster.
I believe too much emphasis is placed on the importance of some IMC training. You are either VFR in a VFR machine, or instrument rated in a IFR machine. A little knowledge can be a dangerous thing.
Just think, you don't take the "go lower and slower" rule far enough, and you get into IMC, what height are you going to be at? Less than 500 ft? Almost certainly - you'd be an even bigger idiot to end up in IMC higher than this. So, you're now IMC with the ground less (probably much less) than 500ft away. Assuming you can even do a big turn on instruments, the chances of losing 500ft in the process are pretty high. You may get lucky and control the airspeed and vertical speed well, but you've got every chance of boring a hole in the ground, even assuming you emerge a hundred or so feet above the misty ground.
Given the same initial situation, ie getting into IMC low level, (assuming ice is not an issue) our IFR pilot/machine would climb straight ahead at max IMC rate, get to MSA, and then decide where to go to get down again. Assuming he did this, our VFR pilot has no approach plates to refer to, no experience of making an IMC approach, probably limited fuel and a massive amount of anxiety. A pretty lethal mix.
Night poses greater risks of going inadvertent IMC, but then you've got to be much more cautious. Only follow routes with reasonable ground lighting, keep high, and always be wary of your visibility decreasing. If you could see a town 7 kms away and then it disappears, a big chunk of cloud has just got in the way. Descend until it returns in sight, then think sensibly what to do. Intermittent use of landing light to check for moisture is very valuable.