Yeah that helps, thanks. But my concern is that if I read the question, and then only halfway through it I decide to diagram, I will lose valuable time rereading the question. I was wondering if there's some type of clue (perhaps by the type of question) that would make this decision easier and/or quicker. I think by reading the question stem before the stimulus may help on that.
Your question is about an issue many students commonly face early into preparing for the LSAT and one worth asking.
Diagramming is useful for some question types and not for others when you are taking a timed test.
You must keep in mind that many of the various diagrams in LSAT prep books and the ones instructors write on the white board during prep classes are meant for educational/illustration purposes and not always meant as an example of what/how much you should diagram on test day. Many of them are meant to illustrate, teach and drill in the concepts and relationships that exist in a given LR question.
For instance, on test day/under timed conditions, it is not very useful to diagram Main Point/Main conclusion questions because your task is simply to identify the main conclusion of the argument and find the answer choice that restates/paraphrases it. Same thing with role in the argument questions. Even if there are conditional statements, diagramming them out is not a very efficient way to determine if a given statement is a premise, counter premise, sub-conclusion, or main conclusion.
The LR question types that diagramming sufficient and necessary condition relationships when presented in the stimulus and/or answer choices is most useful for include:
must be true/most strongly supported
Must be false
sufficient assumption/justify the conclusion
parallel reasoning (non flawed ones, but sometimes with parallel the flawed reasoning ones if the flaw is based on conditional logic)
Principle questions (there are several variations of these)
Flawed method of reasoning (Not all, only when the argument is based on conditional logic)
Strengthen and weaken questions occasionally, but not that often.
Regarding wasting time re-reading the stimulus to make your final selection between two answer choices, that is mostly a mythical fear. In reality, most students that have trouble finishing LR sections in time waste a lot of time debating answer choices
BECAUSE they didn't fully comprehend the stimulus and/or overlooked/forgot some crucial details that make the difference between the correct answer and the most attractive trap answer.
This is especially true with the higher difficulty rated questions that many test takers answer incorrectly. A quick re-read of the stimulus once you have it narrowed to two (sometimes three) contender answer choices should take no more than a few seconds since you have already read it and are already familiar with it. Refreshing your memory of the finer details and nuances of the stimulus can make the difference between getting the point or selecting a trap answer and typically takes less time than people spend debating answering choices on hard questions they get stuck on.