I've voted for 5.10 as that is my default...

However, it is a bit dynamic as it will change depending on the code. If it is a very simple module from a code perspective, I'll wind it down, whereas if I need more recent features, I'll wind it up.

But - I never specify above 5.16 as quite a few shared hosting providers are stuck with this version of Perl and I mostly write modules to run on webservers.