I have a full set of 0.3, 0.6 and 0.9 soft and hard grad ND filters as well as a 0.6 reverse grad ND & 0.9 reverse grad ND filter, which I can combine with a 10 stop ND and/or a 105mm Pol filter. So it is easy for me to compare the effects.
The 0.3 grad NDs were a waste of money. I never used them so far. The same goes for the 0.9 reverse grad ND filter, which is very rarely used.
For an average sunset or sunrise I use only the 0.6 reverse grad ND filter when the sun is near to the horizon. Very seldomly I need to combine it with a normal grad ND (at the sea for example). The attached shot is also such a case. It was shot with a 0.6 grad ND + 0.9 reverse grad ND in order to see something of the village on the bottom of the picture. I used the "expose to the right" technique and then made a HDR from the single RAW file.
If you don't plan to buy a dozen filters, get a 0.6 hard grad ND and a 0.9 soft grad ND filter. A pol filter is a very useful addition to the grad ND filters. The 105mm Lee adaptor ring works well but any 105mm pol filter used on the Lee filter holder will add some vignetting to wide angle lenses. With my TS-E 24mm L it starts at about 8mm of shift when using the 105mm pol filter, no matter if a grad nd filter is used or not.