A clear "It depends".
The METAR is nice to see, but it does not say much about the amount of deposit accumulating on the wing. The best way to find out about what is or is not there is still to get a ladder and climb on the wing to see and feel for Yourself - some ice is only detectable by putting the hand on it.
Some points about estimating the probability of ice accumulation:
-What is Your fuel tank temperature? If You carry lots of sub-zero fuel, beware of fuel-induced ice on the wings that may occur even with well positive temperatures in precipitation. One tell-tale sign may be frost on the wings lower surface.
-The METAR reports some wind. It may well be that the snow is drifting and not sticking to any surface in high wind conditions; mind the little stagnation corners on the leeward side of the fuselage e. g. and also turns during taxi when deciding not to deice.
-What is the weather doing in the longer term? If it is only slightly snowing, but there are dark clouds approaching, remember that deicing will take several minutes and select the required hold-over time and thereby the required fluid mixture with this in mind. A fluid selection that will get You airborne safely with only the odd snowflake falling will see the HOT run out in a flash in heavier precipitation.
-Type dependent, there are other tell-tale signs. On the DH8, when You turn the propellers and find deposit on the blades, You can be certain that the wings look the same. Deposit on the fuselage may be an indication too, but if You see nothing on the fuselage, remember that the cabin is heated unlike the wings. Again, climbing on the wings is the best way to find out what they look like - no need to speculate.
Of course, there is no doubt when it comes to deicing - if You are unsure whether to deice or not, decide for safety and have the plane deiced.
With regards to Your second question:
The selection of a 1 or 2 step procedure depends on whether there is deposit on the wings. Basically, the 1st step is about washing off whatever may be contaminating the wing; for that, hot water with some rather highly diluted fluid is used (refer to tables for exact selection). After this step, the wing is clean, but the HOT is typically nil to rather short. So if there is no precip., You are good to go. But if there is precipitation present, in a 2nd step a (nearly) undiluted Type 2 or 4 liquid is sprayed on to protect against the new contamination and get a usable hold over time. Again, refer to Your companies tables for fluid and mixture selection.