Andrea,
This is maybe just my personal opinion but to me getting the values correct and the texture correct is more important that perfect hatching lines, like Brenda does, for example. At least I hope so, because my lines are little quick strokes or squiggly marks. My hands aren't as steady as they used to be.
And remember if you smudge a little you can get a smoother texture. For example, if you find you have a hard edge and it should be a smoothly rounding surface then smudge back and forth across the edge with a Q-tip gently to soften the edge.
From your examples and from looking at your gallery, it seems to me that you have good control of your lines. So my advice is to study the texture you wish to imitate and concentrate on getting the full range of values. Hope this helps.
Bob.