When deriving alternate minimums do I always round the ceiling listed on the approach plate up?

If not, when do I? And how do I prove it either way?

Thank you in advance.

Technically, no. But in practice yes. The Ops Spec simply states to add 200 or 400 feet, but since the ATIS only reports to the nearest hundred feet, then you end up having the round in the end.

I've seen some DA be 407 feet, for example. Would I still round up to 500 feet?

Technically you just add the penalty to 407', say 200' for two approaches so your minimum forecast ceiling for filing would be 607'.

As 2Stg pointed out, WX reports and forecasts are in 100 foot increments, so a 600' forecast would be 7' too low to legally file. So you'd really need a 700' forecast.