I have always taken the view if CB's are around so long as you can see them and stay away it is not too big a problem. Just how far to stay away is the trick! The real problem of course comes when they are embedded, and this makes me very nervous flying IMC without weather radar, unless there is little risk of embedded CBs. I think your instructor was very wise - at some point if you fly enough you will see lightening or at least the warning sounds on the VHF. At least you will have a better idea of the sort of distances to be away from the storm cell(s) and yet be able to continue your journey - or to turn around!