My wife has 8 federal student loans, and they just got transferred between servicers. Under the old servicer, they were grouped into two groups, A and B, with multiple loans with different rates per group. We were paying off the group with the highest average interest rate first.
The new servicer splits out all the loans separately, so now we can tackle the highest rate ones directly, while not paying off the lower rate loans within the group. The thing is, I bet we could have done this all along, instead of limiting ourselves to the group level.
So, if you're in the same situation, learn from our mistake: if you have loan groups, see whether you can break them out to pay the highest rate loans, maybe by a letter and a check in the mail.