This is just general info...
When new at converting... use a coffee filter... when your a little more experianced, Then you can try it without... But make damn sure that all the binders have completely fallen out of suspension. This is one of the reasons, when electing not to use the coffee filter,that the "new to converting people" have problems.. Not letting it sit long enough.. The other is trying to get every last drop out of the vial and sucking up some sludge off the bottom...
Just because your an old pro at it... Doesn't mean evry new guy will have the same outcome.. they won't. It's irresponsible for you to think so and tell new guys not to use the coffee filter...
The BA is in there to fight off any bacteria.. So unless you make this in an outhouse, chances of you really contaminating the gear is very slim.
Worried about shit falling into the funnel while filtering... Saran Wrap... enough said.