Do I need to thin paint?

This very much depends on the project you are working on.

For painting the base coat I would say no it is not necessary as the paint should be of the correct consistency to paint straight on when using a brush, as long as you have given it a good stir or shake depending on the type of container the paint comes in.

If you plan to add a wash to your model you will need to thin the paint. You can get ready made washes from most paint manufacturers. These have been diluted and will save you time on the thinning process.

If you plan to use an airbrush you will definitely need to thin the paint. Although you can get paint specifically designed for use with an airbrush.