I think it depends what you would engrave. On the back of a watch would be ok as no-one will see that when he wears it, but on a bangle, if it can be easily seen, will your mum wear it again? I've not got anything engraved for my BMs as I wanted them to be able to use things again, not just the one day they are my bridesmaid.
I will get it engraved on the underside of mum's bangle, so she could wear it again without it having a message on show. I know she won't probably wear it on the day anyway as it is silver to match her Mackintosh style silver cross which never comes off, but she will probably pick out gold jewellery for on the day as her MOB dress has gold tones.
Hopefully my bridesman will wear his watch on the day or some other time, but if not then I know his partner will wear it as he helped me choose it! And 'thanks duck' luckily applies to both of them