What an odd rule. Or maybe it had to do with a specific type of dishes?
For example, I love to cut garlic cloves into 2 to 4 wedges per clove, make small cuts into a leg of lamb and insert the garlic wedges, then roast or grill the leg of lamb. You can even do that with a rib eye roast (although I've never seen it done here in the U.S., it is common practice to do that in France).
I'm not too sure why or when I use crushed vs sliced vs minced, but I use all those forms and more. I don't think I've ever mixed and match several techniques in one same dish, but why not, sounds like a good idea.
What I do more and more with many dishes is put the garlic near the end of the cooking, rather than in the beginning. You'll get much more garlic flavor if it is nearly raw. Even if you add garlic at the beginning, add a little more toward the end of the cooking for added flavor.
A typical example is ratatouille, where garlic is added toward the end, a few minutes before you stop cooking your ratatouille.