I would suggest only doing the roast. This will remove the the sulfur and will make the assay much cleaner. Unless you have very high concentrations of base metals (copper, nickle, etc) you should not have a problem. Some of the base metals will report to the slag, and some will collect with the lead and be absorbed into the cupel.
If you have very high base metal concentrations you can reduce sample size, or add extra reducer to drop a larger lead button and use a larger cupel- or both.