Puppies can start heartworm meds w/o a blood test, since they're puppies and haven't even been around long enough for heartworm to develop (at least, I think this is the reason) so you should start heartworm medicine soon.
But I think blood work is always good. I wish I had done it when Jackson was younger so I had a 'baseline' idea of what his 'normal' is, as he didn't have bloodwork until he was sick and I didn't really know if it was him being sick... or if it was just his 'normal' if that makes sense.