Long before I started doing contests in ham radio, I got used to "sharing" frequencies when propagation meant the bands were active. Propagation would shift and all of a sudden another QSO would become audible right where I was rag chewing with someone. The Q signal QRM was created because of hams interfering with each other.
In recent years, especially since JT65/FT8 became popular, the bands only seem crowded during contests!
For the original poster's question, it is not accepted for anyone to "reserve" a frequency in contesting, though outside of contesting there are many nets that believe they have certain frequencies reserved for long periods at the same time each day.
From the Contesting Aargh FAQ:Is it OK to just start calling CQ on a frequency if I listen and don't hear anyone using it? Well, if you listen long enough that is probably OK. Most feel it is better to send QRL? or ask, "Is this frequency in use?" once or twice. Think about it this way: if you were going to take the last scoop of mashed potatoes from the bowl at dinner, wouldn't you at least ask "Are these potatoes in use?" However, this is one of those religious debates where some sneer at QRL? and others are aghast at anyone who would not QRL? There are rules against intentional interference in Amateur Radio, but there is no definition of what you have to do to avoid causing intentional interference.
I have a follow-up question on frequency use. Yes, go ahead.
There seem to be certain frequencies that you are not allowed to use in contests. What's up with that? Well, on some of the low bands like 160 and 80, there are recognized DX "windows" from back in the day when working DX on those bands was rare and exotic. So, today generally only non-US stations are supposed to call CQ in those windows. There are also other frequencies that are used by groups like SSTVers or pig farmers or bean bag collector nets that tend to get upset if anyone else uses "their" frequency. While the reality is that no one owns any particular frequency, life is short - there are other frequencies to run on and rather than give the haters a reason to hate both the players and the game, courtesy is to just QSY. Also, when there are any emergency operations going on, it is more than courtesy to avoid interfering - it is core to our hobby. AC6V has a pretty good guide here.