Hello! @sansouci I work in the UK and tips generally aren't expected at all, most guests tend to leave tips in the restaurant or the bar, some write us lovely letters and leave them in the room before they check out. Any form of thanks is appreciated it doesn't need to be money :)
No the dirty bed sheets aren't true, well not at my hotel anyway. I've been here almost 3 years and very rarely have complaints about the sheets, we have employees who's sole job is to constantly wash, dry and fold bed sheets, towels, blankets etc.
A good tip would be if the bed sheets have fold lines on them? As In if you iron something and fold it, it may still have the folded lines when you put the sheet on the bed just like in your own home, if it doesn't have those lines and is totally smooth or "worn" looking I'd definitely ask them to change the sheets as that to me is an indicator of those sheets being slept on!
You don't really have to make the bed as it should be fully stripped down after every guest departs for cleaning and fresh sheets anyway, as long as everything is in the one place its fine, ie pillows and duvets on the bed and not thrown around the room, same idea goes for towels and things of that nature! Most guests do try to tidy to some degree but you do get the odd few that leave the place in total disarray