When should I use caulk?
I have come to steal a technique I learned from Mike Holmes, because though I have done plenty of caulking I have to admit that I'm just not very good at it freehand.... I tape the edges of my caulk line.

Then I use a caulking tool for a good uniform shape... The "U" shape is what you're after (more like a hyperbola, though).

Afterwards I pull the tape and have a nice straight caulk line.