Is a Karnaugh map always a good way to simplify a Boolean expression?
Karnaugh map are a very good way to simplify logic expressions. However, more than four variables can get a bit tedious for us humans to do.

Moreover, its very difficult to spot something called "Static Hazards" if you tread down the algebraic simplification path. Designs that are Static Hazard free are very important in digital design and Karnaugh maps can be used to avoid them.

Karnaugh maps will always get you the simplest expression as long as you form the largest group possible even if that means looping(including) ones you have already accounted for.

With regards to your observation, it is wrong. You cannot simplify this to A OR B, look at line one on your Truth Table.....