Also, forget about address classes. They've
been obsolete for over 30 years.
That said, sticking with class sized masks and matching the network
numbers for class based networks tends to simplify addressing and make
things less confusing.
That's only true because people have learned a lot of bad habits
with IPv4. Also, originally, all IPv4 addresses had the same size
subnet, /8. As that allowed only 256 networks, address classes were
introduced, to provide more, smaller networks. Then even that
proved inadequate, so Classless InterDomain Routing (CIDR) was
introduced which is what we've been using for decades.