Interesting questions. I've just sat and gone through my overstuffed lever-arch folder full of certificates and documents for my ancestors and was surprised to discover very few x's marked on certificates since 1837, just a couple early on in the 1840s by ancestors who were in rural areas. I'm wondering if this is because most of my family were in and around London and had more access to education - enough, at least, to sign their names.
I was pleased to note that both my 3xgreat-grandparents (who were both 'deaf and dumb from birth' according to census returns) signed their names on their marriage certificate in 1842. I know that my 3xg-grandmother attended a deaf school (I have the school record) and suspect that's where she met my 3xg-grandfather (the school were unable to find a record for him, but the records are incomplete).
The earliest signature I have is on a copy of a will signed by my 5xg-grandfather who was born c.1719 and died in 1807.
My earliest Jones is my grandmother, and I've been lucky to be able to trace her line back as far as my 3xg-grandfather born c.1801, with my earliest certificate being his marriage in 1841. I remember when I phoned the GRO to order that particular certificate, the woman I spoke to on the phone was very impressed that I had managed to get that far on a Jones line, she actually said "well done"! And, my g-grandfather Jones, his father and his father were all called Thomas just to make things nice and simple for me

I haven't yet found any Smiths in my tree.