Many people feel they simply must grade grammar in a paper. It is usually not an obsession of mine, but if you insist, go right ahead. Make it one of your dimensions and decide how much weight to give that dimension. But once you have graded that dimension, do your best to grade the other dimensions independently of the grammar that obscures the student's thinking. Otherwise, you punish them twice for a single failing.

There will be close calls. Perhaps the student's presentation makes it just barely out of the basement of achievement, but you are not comfortable giving the benefit of the doubt. The answer herer is to establish a regular rules for these cases and always follow it. On rules might be to split the points (give a .5 if it is between 0 and 1). Another might be to round up or down. Pick a rule a stick to it, and let students know what you did. This helps reduce the agonizing over the partial points.

On occasion, I find a student has done something interesting reasonably well, but not according to the rubric they knew about. Usually I ask students to tell me about this ahead of time so we can negotiate some solution. If the student's proposal is interesting enough (and looks like it will achieve the goals I have for the exercise) I arrange to grade the alternative independently. This is much easier to do in smaller classes.