I've always appreciated the fact that billing my (& my assistant's) time(s) most-accurately reflects the work I/we've done. There is too little that is uniform about the projects/returns/reports I'm tasked with, from year to year; so I do not 'buy into' the annual blurb from one published consultant or another that "hourly billing is passe."
It indeed can be exasperating to have to explain to a client why this year's invoice is "so much more than" last year's, but, every reasonable client can understand that, in the end. (Funny; I can't recall hearing from any client, "Wow, you charged me half what you charged, last year!") Feeling free to jettison unreasonable clients, I nonetheless have seen my practice grow past the point that I could handle without an almost-fulltime assistant, now. I must conclude that "the proof is in the pudding."