This is just an opinion... and I usually run my boat in salt water, so a flush is essentially every time.
Really, you don't know what kind of crap is being sucked through your cooling lines... mud, vegetation, etc. A flush is just good practice. You really don't want your cooling lines to get clogged so a flush of fresh water just cleans things out.
Since you have to spray engine fogger and lubricant through your air intake after every run, hooking up the hose to the back just seems like a simple extra step. And obviously, you don't want to run your engine for anymore than a few 10s of seconds if you aren't cooling it. it usually takes me a minute to fog the engine and spray lubricant in the air intake. So, I always hook up the hose when ii do this.
Des