I've covered the process that I use to shave spindles in an earlier post. I find that following the fiber line closely and taking the spindles through a set sequence helps a lot with both speed and consistency. Sometimes, nearing assembly, I find that the small ends of the spindles are too heavy. This is normally the result of cutting them shorter, which moves the tenon down the taper, making it larger. If I have already moved into the round phase of shaping and see that there is a lot of material to remove, I will reshave the end square and follow the sequence shown in the photo below to quickly bring it to size. The tenon marked 15 is the oversized one and the sequence proceeds to the right.
It is much easier to adjust the four facets of a square tenon than the multiple facets of a round one. While making the curved settee, I found myself with 21 fat tenons, and plenty of opportunity to appreciate the value of the technique.