I need to visit Panera. This thread is making me so hungry!
At least here you're in good company! Oh, and welcome aboard, C'pn.
For me, it varies, depending on the sandwich.
If it's a deli-type sandwich with sprouts, pastrami, some spicy mustard, tomato, and onion on a good whole-grain bread, then it's diagonally.
If it's a meatloaf sandwich, chicken breast, or other thick type of whatever, then it's straight across.
If it's a Reuben, it's straight across. Every time.
If it's PB&J, it's diagonal.
Grilled cheese? Depends on the temperature outside and whether I'm having tomato soup with it. If it's just the sandwich, it's straight across. If it's cold outside and I'm having soup, it's diagonal (makes for better dunking).
Roast beef? It better be on a hoagie roll...and yes, it has a diagonal cut - across its middle.
Yeah...I'm weird.