Peter, When improvising, I find it is very helpful to be very familiar with the melody and its chord progression. I tend to use either major diatonis scales, or blues scales when improvising. An example of each is: Major Diatonic Scale in C: C,D,E,F,G,A,B,C.
Blues Scale in C: C, E^b, F, F# A, B^b, C.

Quite often one can use a blues scale as a substitute for a pentatonic scale. I say quite often, but perhaps not always. Your own sense of aesthetics will tell you when it will work, and when it won't. Use of the blues scale often creates a jazzy element to what you are playing. This is how those Texas fiddlers create Western Swing out of traditional fiddle tunes like Forked Deer, for example, or Billy In The Lowgrownd.

If instead you are playing straight ahead Old-time music, then you may want to limit your use of the blues scale to those country rags, like Natural Bridge Blues, or Hawkins Rag, etc. In the final analysis you need to be familiar with the kind of music you are playing. Trying to be jazzy in a stringband is going to raise some eyebrows, to say the least, but will work fairly well in a bluegrass setting for instance.
Jim