Is it necessary?
Dam, my Opemus III doesn't allows for it; luckily both the neg and the paper are not aware of it! (-:

Seriously, for the maximum light this is true, but I've always used 6x6 enlargers (a stolen Durst 600 and now the Opemus) for 35mm with the 6x6 condensers and today I print 8x10 in about 10s at f8 with a 75W bulb.

Jorge O