This is worrying for me and I certainly do not know the various rules when I go on holiday abroad. I have a watch roll and like to take some of my more valuable watches with me in my carry-on luggage.
Two common reasons for taking a few watches with me:
To differently wear a favourite one in the evenings and other ones during the day plus a cheap Casio for the beach, swim and to wear when visiting the less salubrious locations to avoid attention.
Not to risk a loss if my home is burgled. Insurance will never make up for the loss of a cherished watch as for anything else.
Compared to many on this forum, none of my watches has a current market value in excess of £5,000 and most are valued a lot less, although many of mine being older than 20 to 50 years, a few would have cost somewhat more the equivalent amount when new. Keeping watches in a bank where I live is not an option and for me is pointless because I like to wear/“play” with all of them fairly regularly.
I did amateurishly only once take photocopies of paid invoices for the watches I carried in the hope that if I was confronted by customs officials I could “prove” they were personal property. In future I shall do that every time. Going on holiday my wife carries some of her favourite jewellery; I usually pack more than one pair of shoes, a few silk ties (that are rarely worn!), shirts IT stuff etc – so where does "commerciality" stop with some certainty? Take a boat or private aeroplane overseas and you will carry a customs or general declaration document: but for a spare watch?