I do. I try to do it once a week but because of the rota I work I usually do it when ever I can.
I don't to it everyday, but usually when it is looking mucky of the green stuff starts appearing. I like to see a clean bucket, after all I wouldn't give him "green" water in his stable.
Seeing as Spock drinks it so quickly, didn't get very dirty but I scrubbed/rinsed it out twice a week. When he had the self filling one, I did that once a week (took it in turns with the other girl in my field).
If you are happy for your horse to drink out of a mucky, dirty bucket, then you shouldnt hesitate to drink from it either.
This is one thing we were always told in the riding school where I worked. If you would not drink it then don't expect the horse too. We used to have "helpers" who would try to skip certain duties (like cleaning water buckets, they would just top up). It used to "scare them into shape" when we told them if you expect the horse to drink that havwe a slurp yourself.