I find for simple supports, that I get much more control to just model them myself at the same time as the part file. Exact support where I want it an no waste.
I'm relatively new to Simplify3D and generally pleased with the software. Unlike some others, i'm not to good at design so would find it difficult to add supports to the model in cad as has been suggested by another member so like many others have to manually remove each unnecessary support pillar one by one. Some sort of multiple selection tool to delete in one go would be perfect.
For things I design myself Leeway, this is always possible. But the large amount of the paid work I do involves fabricating and finishing pieces that I had no hand in designing, so I'm simply importing the STL.