This is an interesting thread...
Whenever I visit my aunt - who is 96 yrs old, she always prays at mealtime. I do not pray anymore - haven't for many many years. But, out of respect, I will bow my head and remain silent as she says her prayer.
She will try - from time to time - and ask ME to say the prayer. I decline - as I have told her in the past that I do not do that anymore.
She will politely smile, and say her prayer. (Although, I have jokingly told my girlfriend that someday I may accept and say the only prayer I remember... "Over the lips, and through the gums... look out stomach, here it comes. Amen.")
On occassion, there will be a lot of relatives gather together at her home for a meal. Everyone there comes from different faiths (we're not talking JWs here). Mormons, pentecostal, etc. During the prayers (usually said by my cousin's husband - who are mormons), everyone bows their heads respectfully.
I feel that it is a 'manners' thing.
Yes, when I was a JW - I remember those who would not bow their heads at prayers given by 'heathens'. JWs obviously never learned manners.
Regards,
Jim TX
P.S. These are my observations. Yours may vary.