Here, me droogs, is the skinny on this age-old question:

CONTRARY (and I know this is heresy to speak so of the brewing demigods, but I tempt Fate anyway and repeat:!) CONTRARY to what Charlie Papazian's book says on the matter, it has been proven - both under chemical analysis of the tested beers, and by nationally-certified judges doing side-by-side taste tests that blowing off the foam on the top of the fermentation is NOT beneficial to the beer.

Trust me on this. I've tasted it.

Blowing off does NOT eliminate harsh flavors, and grating bitternesses, it does not "clean up" the flavors of the beer, it does not do anything beneficial to the final product. Quite the opposite. Blowing off LOWERS your hop utilization meaning you get less bitterness OVERALL off of your hops, thus you are wasting money by blowing off your foam. You might as well buy less hops, or use less of them, and not blow out and save your money.

Commercially, nobody wants to blow off their foam. The Trappist breweries do not, the Urquell brewery in Czechoslavakia does not, Guinness and Bass do not, Budweiser and Coors, and McMenamin's and Deschutes and Rogue and Full Sail - none of them want to blow off the krausen/foam. They sometimes have blow-off systems to prevent catastrophic explosions - which is good, and to maximize the volume they can brew - which is also good - but they deal wth a lower hop utilization if they blow-off a significant amount of foam.

If you can avoid blowing out by fermenting in a larger fermenter, do so. We like to ferment five-gallon batches in an eight-gallon fermenter and eliminate all the blow-off.

Also: blowing out is a pain in the neck, blow-off tubes are expensive to replace, you can lose "x" amount of beer boiling off the top of your carboy, it is a mess in general, and POTENTIALLY (although unlikely) it could allow a "bug" to creep up your blow-off tube into your beer since you have established an "inside-to-outside" connection.

If you are still unconvinced, do a side-by-side and brew the same beer both ways. You will find one to have less bitterness and less hoppiness: the one in which the krausen was blown out.

So, what to do? I recommend you primary ferment your beer in an oversized fermenter - ferment five gallons of beer in a six gallon or larger fermenting container. I have seen and used food-grade SEALED buckets with great success, and treated carefully they will cause you no problems (use an airlock on top!), but you can also use six or six-point-five gallon carboys if you are glass-fanatical. See my info sheet on glass versus plastic for more verbiage on the matter. Also see my page on when to rack into a five gallon glass secondary at the end of the primary - I believe this to be critical for the highest-quality beers.