Yes, for safety the steel should be removed along with any crud, water, liquids etc. I know other will say that it doesn't matter that you can scrape off the scum and doss as you melt. But a liquid h2o will expand to a gas around 1,200 to 1,300 times in volume. This also goes for other liquids. the oils and paints can flash into flame.
The problem with steel and other materials is it will cause, problem some minor some major if you are casting ingots or flywheels, from the pinholes, blowouts, to porosity. As the temperature for the liquefactions point will be affected by the material.
Clean and dry will result in the short term a little more work, but a savings in time and fuel and fustration in the long run.
Of course this IMO.
http://www.backyardmetalcasting.com/forums/index.php here is the Granddaddy of all the metal meting sites. With all the links and people doing it everywhere. From a hole in the back yard with a fire in it to a section of heavy walled pipe with a plate welded to the bottom for crucible. For the occasional melting of aluminum.
http://www.theworkshop.ca/casting/casting.htm he does a lot of aluminum casting great info.
http://www.foundry.ray-vin.com/ ;D