Sorry if this sounds a bit "touchy" but...I wouldn't want the stewards to ask me what my salary is....so I don't think it is my business to know what their salary is.
As to whether you wish to tip...your decision. You want to tip, I'm sure they will appreciate it. If you wish to be among the few who choose not to tip, your choice. No one will say a single thing to you about it and the service you receive will remain at the same high level....though you'd only be aboard a few hours past the "normal" time to distribute tips and thank the stewards.