Unless you are going to be building many frames a seattube reamer is an expensive tool to have. Better to just have a shop do it for you. It's also worth mentioning that some seat tubing actually already fits the appropriate sized seatpost without bothering to ream it to 27.2.
Easton Aluminum ultralight tubing fits 27.0 for example. The opposite end fits 29.8. Some of the other sizes of seatpost are a bit harder to find but that is a cheaper alternative.