Start from the default position, which is there is no need to tip everyone as you are already doing that through the daily service charge.
Beyond that, MSC policy is not to tip individually, but of course we know that isn't going to stop people, especially on a cruise out of a US port, so we focus on those that go above and beyond, which narrows the field somewhat.
As explained above, the envelopes are readily available, and although not expected you will not find anyone actively refusing a tip 🙂