If you have the RAM for portage build space in tmpfs, the kernel caching will do that anyway.
By setting it explicitly, you avoid a lot of writes that will never be read. That's good for SSD life.
The switchover point depends on how much RAM you have, how many CPU logical threads you allow portage and what emerge --jobs= setting you use.
Regards,
NeddySeagoon
Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.