ShenziSixaxis
Article Writer
I've found that that is true. As long as I'm already hidden when I drink the potion, they never see me (same with the spell), although there are exceptions. I think it's a combo when using either the potion or the spell. Of course, if you don't have the silent cast perk when casting invisibility it makes the whole thing moot anyhow.
IIRC dual casting it with the DC perk makes it last longer, so no, you can use it just fine without the silence perk.