A paint cleaner can/should be used when you aren't polishing, before polishing, after polishing, or both before and after.
No polishing- Obviously if you are doing a detail and are not polishing, you'll want to remove any old protection or grime left of the paint after a wash to ensure a clean surface for your LSP to adhere to.
Before- The same reason as above, but instead of having a clean surface for your LSP, it is a clean surface for the polish to work on. This is also a good idea to use a paint cleaner before polishing to remove any oxidation that could hinder the performance of the upcomming polishing steps.
After- Using a paint cleaner or cleanser after polishing is also a good idea to remove and left over oils or solvents left by polishes. This will leave a clean surface for your LSP to bond to. Also using a product like KAIO or P21S GEPC before their respected counterparts (KSG & P21s carnauba) may result in enhanced looks and durability, since they were designed to work with each other. Another thing- check out this thread:
http://www.autogeekonline.net/forum...0962-example-why-paint-cleaner-important.html. Beyond popular belief, polishes do not clean paint like a paint cleaner.
Both- As stated above, using a paint cleaner before and after polishing has its benefits. Doing it before polishing will allow your polishes to work better. Using a paint cleaner after, will ensure LSP adhesion and may enchance looks and durability, if using a 2 step system (KAIO, P21s GEPC & wax). This may not be possible on all details however. A 50/50 alcohol or prep sol wipedown could be used after polishing to remove any residues but it won't enhance durability or looks when using a 2 step system.