Here's a picture of a bottom board of one of my hives, hasn't been cleaned by me in several years, if ever. I normally do not clean bottom boards, unless there is something wrong with the hive that has prevented the bees from cleaning it themselves.
Below is a closer up shot, which shows the bottom board cleaned and propolised by the bees, the way the they like it. It also has little nodules on it that the bees build to give themselves something to climb up on to reach the bottom bars of the frames. Scraping or cleaning the bottom board by the beekeeper would remove these and the bees would have to build them again.
Where I am winters are mild, but in places with very cold winters where the bees cluster tightly, it may be too cold for the bees to break cluster to clean the bottom board, in such cases the bottom board may be dirty by spring, and in those situations it can help the bees if the beekeeper scrapes the bottom board clean in spring once the bees are getting active. That would just be a once a year job. But for me, in normal hives I never clean them.
As a slight aside, one of the reasons I do not use screened bottom boards is that bees are naturally very clean, if they have a choice. A screened bottom board prevents this natural behavior, and allows a build up of rubbish below the screen, where the bees cannot access it to clean it.