Hi,
There is a downside with physically checking it. I have two PIX 515 bought 5 years ago with consecutive number like the example below;
FW1 Physical S/N: ABCD1234
FW2 Physical S/N: ABCD1235
But they don't tally with the command line output as shown like the example below (its the other way around);
FW1 CLI S/N: ABCD1235
FW2 CLI S/N: ABCD1234
After checking, I found out that whoever put the S/N sticker made a mistake.
But of course, before putting the device in production, better write down important information like S/N, hostname, location, model, vendor, OS, etc...
Many years ago, vendors put the S/N sticker on top or underneath their devices (any vendor). If you forgot to write it down before rack mounting or putting the devices to production - you will have to borrow a mirror from the dentist. This is always happen as some deployment people doesn't care about anything but to put the devices to production no matter what (with a view of a thrown spaghetti in the rack the way they put network and power cables).
Dandy
