+1 for Sergey's answer. And if you want to apply that to all your StackPanels you can do this:
<Style TargetType="{x:Type StackPanel}">
<Style.Resources>
<Style TargetType="{x:Type TextBox}">
<Setter Property="Margin" Value="{StaticResource tbMargin}"/>
</Style>
</Style.Resources>
</Style>
But beware: if you define a style like this in your App.xaml (or another dictionary that is merged into the Application.Resources) it can override the default style of the control. For mostly lookless controls like the stackpanel it isn't a problem, but for textboxes etc you may stumble upon this problem, which luckily has some workarounds.