Running Cable Underground in PVC Conduit - Do I bond the PVC segments before pulling the wire through?
Is there a simple, definitive answer to this?

Yes
It may seem more convenient to install the wire as you go in a very few cases but the National Electrical Code prohibits it and is quite clear.
From the 2017 NEC:

300.18 Raceway Installations.
(A) Complete Runs. Raceways, other than busways or exposed raceways having hinged or removable covers, shall be installed complete between outlet, junction, or splicing points prior to the installation of conductors.

Most electricians would normally just use a fishtape to pull the wire in after installation. It really is a whole lot easier than trying to stuff the conduit over the wire and then glue it together.
An alternative is a pull string vacuumed into the conduit with a piece of plastic baggy on the end. (Make sure you pull slow and easy to avoid the string cutting through the 90ºs.)
Good luck!