I see this as a matter of personal preference, although it's not likely that doing or not doing it will make or break your ability to do LR questions. Reading the stem first is helpful in that you know exactly what you need to do upfront- which can save some time.
For example, if you know that you are looking for a must be true answer, you will understand that the stimulus is a set of facts and not waste any time trying to find the argument. Or, if you know the question is a main point, you will not spend any time thinking about why the argument is flawed.
On the other hand, some people find that reading the stem first might prompt them to start thinking about answer choices before they are ready and miss key parts of the argument.
I would recommend trying both ways for some questions (maybe trying one LR section with stem first and one Lr with stimulus first) and seeing which feels better. I personally do glance at the stem before the stimulus, but that may be more out of force of habit than anything else. It certainly isn't a bad idea to do the stem first, but it is far from a must- if you find it works for you that's awesome, but if not feel free to go right into the stimulus.
