cyberpan's thread about injecting into a vein made me think of this...
I always aspirate. However, I think if you pull long enough, you're always gonna get blood. Yeah, most of the time you get tiny air bubbles at first but eventually if you keep pulling, blood will inevitably pop in. Am I correct? I realized this one night last year after resticking myself 7 times in the glute because I kept aspirating blood.
Now, as long as I don't get any blood in that first pull, just a tiny air bubble or 2, I plunge.