My thoughts are that it has to do with the
type of chocolate you use. Some chocolate is better than others at melting. And adding a fat, like butter, can give a smoother, creamier texture, but I personally don't feel that adding fat is necessary if you're using a good quality chocolate (like the block of chocolate you said you used).
Now, chocolate chips are sometimes another story, as they sometimes contain something which prevents them from melting readily (which is one reason they hold up so well in a cookie). So, if you're melting chocolate chips for dipping strawberries, you might find it better to add a little butter to the process. Someone else with more knowledge of chocolate and its melting properties may be able shed more light on this topic.
Another thought, what you could do is a little side-by-side test...do a small batch of berries dipped in melted chocolate with butter added, then a small batch without butter added. Decide for yourself which you like better. smileys/smile.gif