It all depends on how clean you actually want it - if you want it looking its best with nice gloss and reflections, its best to clay and polish it.
Or you can just wash it and never bother decontaminating it!
But if you are polishing and sealing its logical to have perfectly clean paint before you do!
If you dont;
Your wax or sealant wont adhere as well as it should.
You could end up doing even more damage to your paint when polishing (if theres contaminates)
The paint wont look its best.
You just seal in a load of crap in your paint, which then makes it even harder to remove!
Its my opinion if you're going to do it, do it properly and then its minimal upkeep and effort when it comes to washing your car in the future
Bertie