When crew in uniform travel as pax, I'll do the usual offer water and move to an exit row if available. Ill always offer a crew tea or coffee to a staff member in an airline uniform, even if it is not ours! Personally, I think it looks cheap when crew in uniform (part of duty travel) have to pay for their food and drinks. I think the company should let us have a drink and a food item. Our airline is one class, but I also think it would look very low cost paxing staff in economy, especially if there is room up past the curtain.
I get slightly annoyed when crew travelling on staff travel start big noting themselves, wearing their IDs and letting everyone know they are crew. Especially when they clutter up the galley! Sometimes crew can be more demanding that full fare passengers!
Generally if I recognise the staff member I will look after them, unless of course they are like the ones I mention above!