I use a 7F or 5F on Morgan when i shave her down for the summer.  She has a cotton coat and when i do this she is smooth, peach fuzzy, and so soft.  I like to make sure everything is even so i go with the short blades.  When i use a 3F or 4F it leave the length uneven (probably not noticed by anyone else but i see it).  And i don't like reverse clipping.      
when i groom silky coated yorkies sometimes i find that a 30 blade and a 4 or 5 comb makes the cut look smoother.      
i think its all about experimenting with each coat.  every coat is different so a 3 on one dog will probably look different then a 3 on another.     
				__________________  Kellie    and    Morgan      |