Not always a must. If it is heat of the moment and going to get my brains fucked out then no, I won't break things up to go rinse. However, if I am planning on getting laid or being on cam at all then I always ensure I flush myself out real good.
Shameless plug for a review I just wrote last night about a shower dong that connects to the shower
This device is going to be my go way of flushing my body from here on out. The bottles, tubes and all that stuff that go into buying a kit is way too expensive and the little fleet bottles from drug stores are way too small. This allows me to be in total control and not have to keep refilling tiny bottles with water.
I have gotten into the habit of drinking Pedialyte after I am done since flushing your body removes good things too such electrolytes and bacteria required for digestion. There are all kinds of methods and recipes for enemas and everyone will tell you one particular way is always better than others. Experiment with several methods and find what works best for you.
No matter how big or small the nozzle going inside you ass may be, there is no such thing as too much lube.
If you are absolutely pooanoid and fret anything ever showing up, keep some baby wipes by the bed. If anything should show simply wipe it of and continue on. Just like with everything else in life, no matter how prepared you are, shit happens.