When I first got my filters I bought a set of 3 Hitech's 0.3, 0.6 and a 0.9. After using them for the last 6-8 months I can honestly say that I can't remeber the last time I ever used my 0.3 or 0.6 by themselves, I have nearly always used the 0.9. I have use the 0.3 or 0.6 when stacking along with the 0.9, but when you do this with the Hitechs you can get quite a bit of magenta colour cast, hence the reason I'm now moving over to Lee. The first two filters I bought from them are a 0.9 hard and a 0.9 soft.
Simon