I do the same as Mac - I use a #12 tip - works like a charm - never have had filling leakage... 
I presume you all are talking about making a dam when using a filling on a cake? Can someone explain this? Is the dam made with buttercream or the filling?
The purpose of a dam is to prevent the filling from leaking out from between the layers. Use the same color icing as your base coat and you can either use a Wilton tip 12 or just the coupler base (without the tip and the part that Screws on).
I make my dam about 1/4 inch from the edge of the cake and then I fill my cake making sure the filling is not higher than the dam. Place the top layer on top of the filling and the dam and lightly press in to place.
Should the icing ooze a bit just take your spatula and smooth it out. If you have any spaces just add some more icing and smooth. I think this step is important. Think of it in terms of painting a wall. If you want nice smooth walls you have to patch any holes and smooth the wall out before you paint.
Now you are ready to crumb coat.
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