If there are frost, snow or ice deposits on the upper wing surface, you must de-ice before take-off, and take notice of Holdover Times according to the level of snow falling or blowing in the wind. You are allowed up to 3 mm of frost under the wing where the fuel tanks are. In addition, the aeroplane can have thin Hoar Frost over the fuselage as long as writing and surface detail can be seen underneath. This is the same as the frost that forms on cars overnight.
It is quite valid to put warm fuel into the wing tanks to melt the ice on the upper wing surface. It will remain wet, but it will not refreeze on the ground for a long time. Once you get airborne, it doesn't matter- you will quickly get up to high speed- remember you can take off into supercooled rain. All that matters is getting airborne and the first minute or so of flight. As long as when you start your take-off roll, you have no ice or snow deposits on the upper surface of the wing.