Oh yes...absolutely.
I do this all the time. It's totally subjective to the person mixing it.... production engineering. Intuition is a MAJOR factor in how these projects turn out. Do I want it sparse and acoustic or do I want a big band sound with Hammond organs and several guitars? DO I want it straight forward or do I want lots of embellished fills and solo's going on? Each tune is different and demands it's own production ideas.
In my tunes, I decide if I want a fill, or if I simply want the rhythm instruments to play and leave the space empty.... I like to keep things balanced.... what I do one place needs a counter-balance somewhere else. For example, in a verse, I try to keep all the fills being played by the same instrument... then in the next verse, the instrument handling fills changes....and in a verse, I want the energy level up....so more things will be playing....I may add the B3 and let the fiddle play low but audible throughout...
Basically, I try to think, how would a band arrange this, given the instruments they have at their disposal.
A good example is to listen to this song...
http://soundclick.com/share.cfm?id=13017714 it's my latest project.
You will hear mandolin fills, fiddle fills, and dobro fills. As things come in and out, I also vary the underlying background rhythm instruments to create a different sound. For example, as the song goes into the mandolin solo after the chorus, I brought the Hammond B3 into the mix, and it drops out again in the verse while a clean guitar is brought in to change the feel.
In a few places, You will hear the piano play the fills.... this is one of the solo-accompaniment tracks... not a solo track but more than a straight rhythm.
NOTE: I write in BB and then use RB to render my real tracks. I then import them into SONAR where I find mixing and editing to be easier and more convenient. I use audio level and panning envelopes to automate the functions of bringing levels up and down. No way could I do that all manually.
If you have specific questions...just ask. It's not a secret.