Clean them up when the threads are dead. When I say dead I mean when a thread hasn't had any posts in a while. Keep in mind that as a device ages thread posts will naturally slow but people new to the device, however they get it, are still interested in reviews and pictures taken with this device stickies. Leave them there they will die with the device forum. OS updates to devices need to be removed whenever they are no longer relevant. Threads like review, first impression, and frequently asked question stickies need to stay stickies. The FAQ I wrote quoted above got me a thank you yesterday. These threads even though they don't get a lot of responses are still valued. Never remove any of the device information stickies from any of the device forums. My personal goal is to never have more than 5 stickies in any forum area. If you look at the L920 forum we have 6 but at this time they are all still necessary.
Those are my thoughts on stickies I am open to debate the subject let me know what you think.