Setting them to low priority if you don't really use them shouldn't result in a noticable difference due to the fact you have many other services running at or above the level your applications are running at. If you notice a difference by doing that you have seriously bogged down the CPU with what you have running. Also many tasks will cause problems when set at too low of a setting.
However if you have background tasks that really seem to make your computer sluggish, setting your current task (Word, IE, AutoCAD, etc.) to Above Normal will help restore the application response due to the fact it would then be running at a higher priority than the majority of the rest of your system and your background tasks.
Word to the wise, setting priority to Real Time can cause problems on many systems.