Electrical – Wire Gauge of Stranded Wire

A few reasons:

• Stranded wire is more expensive to make.
• At a given wire gauge, stranded is going to be larger than solid wire (it's the cross-sectional area of conductor that counts and there are going to be some air gaps between conductors with stranded). This could make a big difference if you have several cables in a limited space like an electrical box.
• The main advantage of stranded is that it's more flexible. You generally don't need this in home wiring because it's all put in place once and hidden behind walls/floors/ceilings.
• When you screw a solid wire into a switch or receptacle, you can tell if it's secure. I could see individual strands coming loose as you fold wires back into an electrical box.

More information in Wikipedia: http://en.wikipedia.org/wiki/Wire#Solid_versus_stranded

I sell a lot of stranded wire and I've never heard of anybody tinning a wire that goes on a neutral or ground bar. AFGI & GFCI breakers come with stranded copper and is rated to fit into these bars, along with solid. I would just make sure the hole you are using matches up with the wire you are using. Most of the ground and neutral bars have different size lugs with different size ratings, like 14 to 10 awg, or 14 to 4 awg or 6 to 2/0 awg.