I have seen when people lose a lot of weight and their skin is loose afterwards. I'm wondering how high is the absolute highest body fat % you should let yourself get to while bulking so you don't stretch out your skin to much.

I am not a fata$$ - about 15-18% bf and I haven't started back in the gym yet. according to this thread:

http://forum.bodybuilding.com/showthread.php?t=579830

I am a Mesoendomorph, and I can bulk pretty easily. I'm wondering if I should do some cardio and lower my BF to ~10-12% and then start bulking. I do NOT want to do cardio while I'm bulking.

The workouts and the new muscle should take off some of that fat.  You can play with diet to make sure it does.
 
20 minute walks are good for low intensity work for just fat focused exercise while you are bulking.

ya. you should see some fat some off while you are lifting.. By the way, just because your weight isnt changing doesnt mean your not gaining a lot of muscle because i lost some fat while gaining a lot of muscle and my weight mayb went up a pound. so just a little heads up  

8/18/05 - 175lbs - 20%BF – 140 lbs LBM – 35lbs BF
Now:
2/6/06- 176lbs - 15%BF – 150lbs LBM – 26lbs BF
 
Just look at the stats.
Nearly the same weight.
10 pounds more muscle and 9 pounds less fat.
 
Use calipers to track your body composition over time.