@Robert Kozak: The reason why I do not like to use IValueConverter is that it is a performance killer in WP7 lists. The scrolling is already not very good there, so adding a converter between the property and the value will cause the scrolling to be even more jerky. On the other hand, if you do Silverlight 4 only, then using a BooleanToVisibilityConverter is a bit cleaner I agree.
Both Glenn Block and I (who are pretty strict about the code) agree that adding Visibility and avoiding the converter is acceptable if it provides a better UX. But I totally agree that if it causes an issue when porting the code, then you should use something else. As usual in software engineering, the correct answer is "it depends"