Say, a pilot is piloting his/her jet commercial airplane and suddenly see cumolonimbus or a dark cloud. Then, should the pilot reduce his/her airplane's speed to avoid turbulence?
What you're worried about is hitting turbulence. The word "impact" isn't typically used, although hail impacting an airplane has on relatively rare occasions damaged airplanes in flight and even more rarely has brought them down. My experience was that it just makes a lot of noise on the windshield.
Your question title is more inclusive than the body. Taking the narrower question in the body first— yes. If a pilot is expecting or experiencing severe turbulence, he should lower the cruising speed if that speed is above the recommended turbulence penetration speed. And in lesser degrees of turbulence, some might choose to lessen the speed if they believe that doing so will give passengers a less uncomfortable ride.
Now as to "any moment" and interpreting that as "any time", there are a few instances I can think of offhand.
- I knew a few pilots that, when they were paid by the hour, would sometimes reduce speed, although I think that they would usually just not bring the aircraft up to the normal cruising speed rather than get that speed and then slow. To me that was unprofessional. I flew with one such captain a few times, and on my legs if I put the airplane (747-100/200) at to mach 0.86 for cruise, he'd ask for it to be slowed.
- In specialized and rare instances, it might be desirable to reduce your speed to be a little more fuel efficient. For example, JFK to Tel Aviv in a 747-100 would usually require a redispatch at a fix abeam Athens. To be legal you had to have a certain minimum fuel at the redispatch point before proceeding to Tel Aviv, otherwise you'd have to land at Athens for fuel. So, if approaching the redispatch point — say an hour or two away from it — you might choose to slow to have a little more fuel on arrival at the redispatch point if you thought not doing so would result in a little less fuel than required.
- Then there's the instance where you know you're going to be thrown into a hold on arrival at the destination so you may as well slow a little getting there, burn a little less fuel, have to hold a shorter time. You have to be careful with this one though, because it might mean others get into the hold sooner than you would have, and all you had done is put yourself farther back in the line than you would have.
- And, of course, ATC may ask for a cruise speed reduction to minimize excessive flow at the destination.