Reaming the seat to fit the blank is the preferable method, but cutting and plugging the rod butt with a "stent" causes no real issues when done correctly. In other words, gluing it in there really well.
In the case of relatively large diameter fiberglass blanks used in conjunction with modern wood insert reel seats, the stents are arguably easier than removing a TON of material from the insert. The observant builder will note that vintage glass rods rarely have wood insert seats, opting for larger diameter, hollow metallic seats.
I'll add that i do not like the solid glass rod stock that Proof sells for this purpose. It is overkill and fairly heavy. Instead, I've purchased tubular glass or cabon stock for this purpose the handfull of times I've gone this route.