Often you cannot replace a Label with a TextBlock as you want to the use the Target property (which sets focus to the targeted control when using the keyboard e.g. ALT+C in the sample code below), as that's all a Label really offers over a TextBlock.
However, a Label uses a TextBlock to render text (if a string is placed in the Content property, which it typically is); therefore, you can add a style for TextBlock inside the Label like so:
<Label
Content="_Content Text:"
Target="{Binding ElementName=MyTargetControl}">
<Label.Resources>
<Style TargetType="TextBlock">
<Setter Property="TextWrapping" Value="Wrap" />
</Style>
</Label.Resources>
</Label>
<CheckBox x:Name = "MyTargetControl" />
This way you get to keep the functionality of a Label whilst also being able to wrap the text.