See, here’s where we are struggling with your question — we think the recipe for chocolate-covered strawberries calls for some quantity of chocolate and some quantity of strawberries. We don’t think substituting one non-ingredient for another non-ingredient is a very good idea.
Apparently, though, you have a recipe that suggests thinning the chocolate with some shortening. The reason you would not substitute butter for shortening is that butter includes some water — enough water to cause the chocolate to “seize,” or harden and form lumps. Do not substitute butter or margarine in any melted-chocolate recipe that calls for shortening. You can, however, clarify the butter (remove the water and milk solids) and then use what is — like shortening — pure fat. That will thin the chocolate without causing it to seize, but it will also affect the flavor.
But (and this is just our humble opinion) we wouldn’t use a recipe that asks you to thin chocolate with shortening. It doesn’t benefit the flavor or texture. Make the effort and commit the money to buy a couverture chocolate that is wonderfully smooth and runny when melted. When you’re preparing something that has (or should have) only two ingredients, those ingredients should be special!