Yes. When there is more than one radical, you first need to identify the primary radical. The convention is for the primary radical to be determined by its placement: (1) top, (2) left, (3) bottom, (4) right.

examples:
>薬  

"Grass" (kangxi #140) on top. "Tree" (kangxi #75) on bottom. So, two radicals, but you use the grass radical to look it up in a dictionary.

>好

"Female" (kangxi #38) on left. "Child" (kangxi #39) on right. So, "female" is reference radical, but there still are two radicals.

>薄

"Grass" (kangxi #140) radical on top. "Water" (kangxi #85) radical to left. Kangxi #41 ("thumb / inch") radical on bottom. So, the top radical is most important. The reference radical is "grass".

Organizing kanji is already difficult enough. Making a kanji such as 薄 referenceable by all 3 radicals would just be untenable. The (top / left / bottom / right) convention solves that problem. Of course, there are exceptions to the convention such as kangxi #163. Don't ever give-up until you look under kangxi #1.

**Also**, the number of strokes in a radical enhances its precedence... yes. it can be very frustrating using kanji dictionaries such as *Nelson*. People these days just use handwriting recognition. For example, one dark day I was brought to tears looking for "音". I kept referencing by the smaller radicals I saw. But in fact the whole kanji is a radical by itself. You've got my respect for using dictionaries instead of handwriting recognition!

**final addendum**  
I just came across a perfect example of how radicals can drive you crazy:  
>意

The more complex radical (9 strokes) that is in the top position, **音**, is somehow *not the reference radical*. Rather, it is the simpler radical on bottom, **心**. You gotta love it.