Chain Bar "Dressing" tool. Is this necessary and used by professionals?

[doobin]

Milwaukee powerfile with an 80 grit ceramic belt. 
 

 

[doobin]

18 hours ago, Muddy42 said:

I've never used one.  I generally clean the bar first in soapy water.  Then I use a flat file to take off any burs, then I have a good look for squareness with a mini set square and flatness with a piece of aluminium. I also use a magnetic spirit level on the bar, knowing my vice and workbench are perfectly level. I bang dings with a hammer. Finally once flat, I remove the tracks in the rails where the teeth run.

With respect, a little torpedo level on the bar on the bench won’t tell you shit. It’ll make you feel good but that’s about it. 
 

Steel straight edge is what you need. A peice of aluminium is liable to be bent from the off unless it’s an extruded profile. 

[Brushcutter]

A bench belt sander is also really good for dressing bars. At least short ones.

[outinthewood]

Nothing wrong with the item described by OP but as Steve B points out maybe not a great file ...

I have yet to meet a human with a eye good enough to give the correct angle with just a flat file on a worn bar, however an old method was to use flat file with two bits of lets say 2"by1" which would show up any real bad left or right inclination. So draw the file supported on said 2x1s with thumbs back towards you and pretty good results. Not so mad using angle grinder with flap wheel with very gentle force.

As for the "don't bother since the rails are splayed", if you are really stuck I have for years put a .063 chain in badly worn .058 bar for the likes of brashing and even felling