I don't stock up on pretty much anything until I've tried a few. Generally I try the toro and the CG, as those are two of my favorite vitolas and will give me an idea if I prefer the blend in a larger or smaller vitola. If there's not a lot of difference, I stock whatever I feel like; if I prefer one over the other, I may try another in an even more pronounced vitola (IE petit corona or something in a larger rg) to see what the difference is there.
One general rule of thumb I use, is that I tend to prefer slightly smaller rg for maduros. I find that the strength of flavor from the wrapper is what I like about dark, earthy nicaraguans, so I tend to want that flavor more concentrated, and therefore enjoy the smaller RGs more often. When it comes to more complex, medium bodied cigars (for example, Cornelious or El Gueguense), I prefer the larger rg, because it provides for better balance and nuance. There are exceptions to this rule (I prefer the short churchill for Sobremesa, a medium-bodied cigar, and I prefer the Tenessee Waltz to the Paniolo, for example), but it's a general starting point for me.
It's all guesswork, but the journey is half the fun.