If you are talking at NPS/USFS campgrounds (ie campgrounds with numbered sites for a few $ per night but usually no hookups of any type), then the tag you put on the post and the number of nights you paid for is typically sufficient. If you want to save folks from looking at the tag, or don't want to have to pack up, then leave your camp chairs out.
If you are talking about dispersed camping on USFS/BLM land (ie pulling off onto a spur and setting up camp with no numbered sites, no fee etc) then 'reserving' a camp site is not really good etiquette. If you leave the campsite, then it is really fair game for anyone, and you can find another when you are done. Particularly in popular areas there is an increasing trend of folks 'reserving' dispersed campsite for the weekend or a holiday by coming up during the week and dropping their trailer, or setting up a tent, then coming back on the weekend/holiday (sometimes many days later or with trailers, weeks later).
It could be argued that it is a little different if you are just heading out for the day, but someone else to come a long doesn't know that, and may end up setting up camp closer than you would like. If you are in a remote area, then it shouldn't matter - just come back to the camp site later, and if occupied find another.