PHP Postgres: Get Last Insert ID

I found a couple of other questions on this topic. This one...

mysql_insert_id alternative for postgresql

...and the manual seem to indicate that you can call lastval() any time and it will work as expected. But this one...

Postgresql and PHP: is the currval a efficent way to retrieve the last row inserted id, in a multiuser application?

...seems to state that it has to be within a transaction. So my question is this: can I just wait as long as I like before querying for lastval() (without a transaction)? And is that reliable in the face of many concurrent connections?