The slot design may be used if appropriate - that is, a small lesion. If you have a deep lesion past DEJ, then you would have a difficult time establishing a minimal outline that a slot design usually entails, so a traditional design would be more appropriate.
I attended a recent WREB exam as an official observer and can assure you that there is no bias from WREB about using the slot design. Your best reference is to use the WREB Candidate Guide for the specifics.
http://www.wreb.org/dental/dentalPDFs/2Operative06.pdf
Contained in this are the Scoring Criteria Rating Scales for the preparations. Amalgam and composite are on pages 14 and 15.
WREB allows one slot design of the two Cl II procedures. It is your choice. Do what you are familiar with doing, but follow the WREB criteria. There is no penalty for doing a traditional design for both cases.