It just does not seem correct/proper to terminate the vent line by just having a piece of Tygon hanging out the bottom of the cowl.
I know you asking more about whether or not to have a filter, but what I do for the vent line exit/termination is use the same thing as for the filler -
http://www.slimlineproducts.com/onli.../f1_fueler.htm. I run the vent out through the bottom of the plane just ahead of the fuel tank as opposed to running it back up into the cowl.
Dan