I have a song that the band I'm tracking used a click to play to, but it's not 100%.  I won't get into the process of tracking this particular band, but editing them 100% to the click isn't an option.
 
But to sum up, the song has a click, but not every hit is 100% in the grid.  Not a huge deal really.  The play well together and the click helped keep them in line during exciting parts.
 
If I want to set up a softsynth midi tambourine to play along to choruses or interludes, is there a process or workflow you guys more experienced would do with audiosnap or something along those lines?  I don't want to record a person playing it because I'm looking for a certain sound that I know I can get with virtual instruments.

I'm the opposite! personally I'd play and record a real tambourine, for me live percussion is one of those things that distracts from quantise and rigidity rather nicely..

Generally speaking, your options are:
 
1. Snap the timeline to the actual tempo using Audiosnap's Set Project from Clip (automated but requires a lot of prep) or Set Measure/Beat At Now (manual, but precise and predictable), and then work to that matching grid.
 
2. Enable Audiosnap on an appropriate drum track, correct any issues with the transient detection and marker location and then Add to Pool and quantize a roughly matching tambourine part (i.e. recorded in real time from a MIDI controller) to the pool or Show Pool Lines and draw the part in the Inline PRV.
 
The devil is in the details. Pick your poison and someone can provide more specific direction. A lot depends on whether you intend to play the tambourine part from a controller and just need to be able to tighten it up to the existing tracks or whether you want to be able to "draw" the MIDI for it. 

I was hoping to be able to lay down a simple pattern using the step sequencer, and then using audiosnap to tighten to to the existing drum track.  Perhaps by using an overhead mic as a guide.
 
The part is fairly quick and would be really tough to pull off using a controller.

And yes, I normally try to play these things live.  However, I have specific sound in mind that I can't get with the instruments I own.

I think if you have Sonar build the tempo map from the actual detected tempo, that you could then apply the midi tambourine effectively.
 
I did something similar when building a midi version of a Steve Morse tune (Night Meets Light).  The basic tempo was set to 120, but there were slight variations throughout.  Once Sonar built the tempo map from the actual detected tempo of the song, everything lined up perfectly.
 
Hope that helps, 
 
Bob Bone
 

I think I'll have to try this to see!  I'm sure it won't be very easy.  But I have a few options here!