If you put a warning at the product level, it displays on a pop-up when the product re-draws.  
But, the same is not true for subs and nested subs.  If you put warnings in them, they only display when you create a W.O.  It would be great if they would work the same.

Currently, as a work-around, we added warning messages into our 2D plan tokens, which will print in Model Space, right next to the product in an obvious place.  This accomplishes the job of alerting the drafter/engineer to the issue, but these 2D warnings don't make it into the database/error log to be accessible in the database.  We have triggers built into our system that recognize the database warnings and don't allow jobs to complete until they're fixed, so using the true "Warning Messages" would be preferrable to the 2D text.