Frankly, I don't see any downside to working as a phlebotomist or med lab tech. If you start early enough to get through the training, you'll get to do some really interesting things. I'm a lab tech/phlebotomist now at a fertility clinic, and I think it's a great experience - you interact with your patients who will be from all stratas of life, get used to doing actual medical work (as opposed to shadowing a physician or volunteering, which is primarily hands-off), and get paid for it too.
Whether it's valuable w/r/t boosting the resume, I don't know. I can't imagine it would hurt: you demonstrate that you're okay working with blood and body fluids, for one. Plus, it's actual patient interaction, as I mentioned before, which I have found to be invaluable (ie, how to read patients as a person, so you know if you can talk with them while you're drawing blood or if it's just a business-like encounter).
If you have the time, go for it! I've enjoyed my experience.