In general, GMATPill recommends taking your weakest section first so you don't run out of mental energy in your most important section.
The only scenario that we would recommend otherwise - is if you are scheduled to take the exam at a time that you are not typically used to "thinking" in complex GMAT terms.
For example, if you are not a morning person - but for whatever logistical reason, you are scheduled for an 8am exam -- it could be difficult for you to focus first thing in the morning. For some people, they need some time to get in the rhythm of things - and that difference could be substantial.
Of course, you want to schedule the exam at a time slot that works best for you and your mental alertness. But scheduling doesn't always work perfectly - so if you're in the situation above - I would say that's one scenario where you might want to start off with something that isn't as important for you -- and then gradually transition into your weakest/ most important section.
Does that make sense?
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