It also depends a lot on the distance of your cast. A short cast may take overweighting by 1 or 2 line sizes to make casting easy. Long cast, such as for cruisers out of a tube, you may want to underweight one size.
I can generally cast a size up or down, but it takes a few casts to get used to the change in rhythm.
Testing your rod and line just gives you a baseline to work from in my opinion and this is just something I do because there is too much ice to fish the streams and not enough to ice fish the lakes yet!