If I may put in my 2 cents here guys:
I see no convincing reason NOT to aspirate. It's easy to do. It helps insure that you don't hit a vessel and inject your gear intravenously. In truth, though, it is unlikely that you will actually mainline a vein and do that--more likely you hit the vein and it bleeds. This increases your chance of a hematoma, and injecting a foreign substance (gear) into that will increase your chance of getting an INFECTED hematoma (abcess).
If I aspirate and get blood back, I simply withdraw the syringe and hold a little pressure on the site, and then choose a different site altogether.