It's best to use solid copper on straight runs then splice into stranded cable for bends and splice back into solid copper to continue the straight run.
(hehehehehehe... does anybody think he'll actually do it?!?)
The type of wire used depends on the application. We commonly use solid wire in house wiring for the most part, but will use stranded for grounding. Although industrially it is sometimes better to go with stranded for moving cable runs or parts. Stranded is more flexible and won't break as easily as solid.