I always recommend running within the rated power rating of your speakers. So, if I am using an 18W amplifier, I do not run it into a single 1x12inch cabinet with a 15W-rated Celestion Blue. I know that some people do, but the speaker is so expensive the risk is not worth it.
Going the other way, running a low-powered amp into a speaker rated at a much higher power is fine. I typically use my SE-5a into my 120W Marshall 4×12 cabinet. Don’t worry that the amp cannot drive these high-power speakers – it’s plenty. (And loud!)
(As a side note, hi-fi is different. There it’s generally A Bad Idea to run a low-power amp flat out into a hi-fi speaker, even one that’s rated at a higher power figure. The reason is that all of energy contained within the distortion is sent via the speaker’s crossover to the hi-fi speaker’s tweeter and that can burn out very easily. For guitar-voiced speakers, with no tweeter there’s no such risk.)