Will Bulk Price Update add a sale price to products that don't have one?

No. The sale price is only changed on products that already have one. Products without a sale price keep showing just their regular price.

So with Apply Change To set to Price & Sale Price, products that are already on sale get both prices changed, and the rest only get their regular price changed. Choose Price only if you want to leave existing sale prices alone.

New prices can never go below zero, and a percentage decrease can't be more than 100%. Use Preview first: its table shows each product's Field, Before and After.