@SethW I think that Cabo has it right on this question. My facilitator told me to only worry about it when mixing trips out/in by car and plane. When flying they always stamp your passport and only get the stamp at the border if flying on the other half of the trip.
I always cross at Mexicali West. Pretty simple to find and they are open 24 hours a day. Here are some good directions I found on Facebook:
For anyone crossing at MEXICALI WEST!! I hear so many reports of people not knowing where to go, or even worse - getting stuck in the SENTRI lane. Follow these SIMPLE INSTRUCTIONS: Pick a point...
Please note:
This action will also remove this member from your connections and send a report to the site admin.
Please allow a few minutes for this process to complete.