I don't know about you, but I wear two fitness trackers every single day. I wear my Fitbit the whole day and my Polar watch during my workouts. The other day I got to thinking, "How dirty is this thing?". It definitely depends on what type of tracker you have and how often you use it for how many germs are actually on it. If you're wearing it to the gym on a daily basis, you sweat on them, your dead skin and body oils collect on them, they touch all of the nasty germs at the gym. It is definitely important to clean your trackers at least once a week.
I would not recommend to throw the tracker in the washing machine, but to wash it by hand. It's important you think about what type of tracker you have and if water will effect it. Most of them are water resistant, however you can never be too careful. If the electronic part comes out of your accessory (like the Fitbit Flex), take it out and let the band soak in soap and water. I would recommend using your everyday dish detergent. If you cannot take out the electronic part out of your band (like the Jawbone Up 24), I would recommend wiping the whole thing down with rubbing alcohol. Rubbing alcohol will get rid of all the germs and is safe for electronics!
Let me know in the comments how often you wash your fitness tracker!