One of the reasons for large borders in archival printing is that degradation from external sources often starts at the edges of the print and work its way inwards. So in that sense, a border can never be large enough. Trimming a print tightly guarantees that when the edges start to go the print is immediately ruined.
Any artwork or photo that is worth anything to the owner, be it sentimental or monetary value, should be checked at least every 5 years and have its mat boards changed. The mat and framing isn't just about presentation, it's also about conservation. The backing and front mat baords act as a sponge to soak up any pollutants before they get to the print. If you don't change them they become a reservoir of pollutants right next to the print and once they are reaching load limit they give zero protection.
To that end dry mounting your print onto a piece of mount board means it can never be changed so you better make sure the mount board is extremely well protected by more mountboard behind it. Or don't dry mount so you can change all the boards on a regular basis.
Nobody does it until passed the time when they should have.
It has to be said that these days people don't have open fires in the house nearly as much as they used to and airborne pollutants are much less of a problem unless you live in a city or busy urban area. Its easy to get overly concerned about being "archival". Besides, the biggest factor assuming you've processed and washed your prints properly is the environmental conditions the owner keeps the work in and that is completely out of your control. You can do whatever you like but if the buyer hangs it above a heat source such as a radiator or on an open fire chimney or in a kitchen with wild swings of humidity, then it won't last long.