I would not pre-heat my Behmor for softer, low density coffees. They will benefit from less aggressive heat application, the slower roast progression helping to develop sweetness without risking the roast getting away from you. These tend to be lower altitude coffees grown below 1500 meters, like Brazil and many of the coffees in El Salvador and Honduras. Also, if you're roasting blends, or roast dark in general, I would also skip the pre-heat step.