False. And for those that install a filter with a catch can or plugged hose end, I strongly recommend you remove and clean the filter every once in a while (maybe every 1,500 miles or so).You NEED a filter on the there dude.
The hose has to be vented somehow, just like the original factory design (it was vented back into the air intake). On a filtered breather re-route installation with a closed hose termination (catch can or some sort of plug), that breather filter is the only path for the pressurized crankcase vapor to escape (those breather caps in the rocker covers are indeed one-way umbrella valves as Lunatic mentioned). Although most of the oil/water vapor from the engine will condense and collect at the bottom of the blocked tube or in the catch can, a portion of that oily mist still suspended in vapor is going to try and exit from the only path it has to escape - the breather filter. That same oily vapor is gradually going to saturate the filter and reduce the filter's ability to "breathe". If left unattended the saturated filter will reduce or eliminate the ability for pressurized crankcase vapor to vent.
In my opinion, the best case scenario if you run a filter and never change/clean it is that you'll someday see an oily mess somewhere around or below where you have your filter located. Worst case scenario is that the over-saturated breather filter element becomes blocked and crankcase pressure is no longer allowed to escape at all and you blow seals or develop oil leaks elsewhere.
Remember- those filters that everyone tends to use are designed as an "intake" filter, to filter air coming into a car motor valve cover caused by vaccuum. Using it for a breather re-route is the opposite since you're essentially using it as an exhaust filter. Furthermore, you're really not trying to filter air as it was designed to do, but are asking to filter oily vapor. Big difference.
I think a lot of folks run those filters because they think it's a mandatory part of the breather re-route or just because they think it looks cool. To me, it's unnecessary and if not managed/maintained, creates a significantly worse problem than the original symptom it was hoped to address. If you decide to use one, just keep an eye on it and clean or replace it once in a while. Maybe every 2,500 miles. The filters I've seen don't specify the ability to be washed and cleaned, but I think you could probably soak them in a bucket of hot soapy dish detergent water, rinse thoroughly and sit someplace warm for a day or two to dry out before re-installing.
Matteson is still correct in that the crankcase pressure has to vent somewhere, so if you don't run a filter, then the only practical option (and is what I do) is to just run your tube down through your frame and allow it to vent/drain to the atmosphere. Simple, provides free-flow venting, and requires no maintenance.
One last thing on the topic- No matter what kind of re-route installation you go with, ALWAYS make sure your tubing is routed so that it continually travels downward so that all condensation is forced to collect at the lowest point at the end of the tube. No uphill curves along the way or you risk liquid collection and clogging where the tubing dips.
Sorry for the long post.