I think 15 mins is a waste of time, personally. I skipped cardio the last time I bulked and it was a huge mistake! Now I do 2 30 min moderate intensity sessions of cardio.
Big Andy69, what was your cycle? Also, what were the results with the cardio added? I know some guys cut out all cardio during bulk cycles, but I'm more into quality than quantity. Even though I plan on bulking, I still have to look good with my shirt off.
It is advisable to work the stomach muscles after, rather than immediately before a workout because they are responsible for your posture among other things and you are therefore more likely to incur a back injury if your abs are tired!