It's probably best not to sink them down to a lower level at all and definitely not significantly down like that. They don't root like tomatoes and are prone to rot. I just give them stakes for a bit, instead. Some people will plant them lower (but not above the cotyledons) with young sprouts. In my experience, there's varieties that will show adventitious roots below the cotyledons and those that won't and the only ones I'd sink even a little bit are the ones that do and only when they're very young.