A long time ago, when I was young and beautiful (three years ago to be exact), I did research with human monocytes. We collected blood from patients. We paid them $25 per 99mls of blood. I even donated a few times myself and started to add the $25 to my monthly budget with a plan to donate each month. Of course, the IRB decided this was coercion and instituted a rule that you couldn't donate your own blood for your own experiments... and there went my Zathan Diesel jeans!
But to bring the issue back to the OP's concerns, you should absolutely do it. Of course, you should be careful that you are not being asked for more than 10% of your total blood volume (roughly 500ml). If you are being asked for 500ml (a la Red Cross), then you musn't donate too often. I'd give it a space of about 2 weeks. Also, if you are frail and tiny, you might feel very light headed. One waspy girl who donated blood for my experiment fainted. We only took the standard 99mls, but she was super skinny. What was my point again....