Your best bet is to have the cat5 already in place from where the ONT is likely to be running to where the router will be. Ask the tech to terminate the cat5 and use it for the router's WAN connection.
Suppose the tech can't or won't hook up cat5, the next best thing is to have the cat5 in place and ask the tech to terminate the ends (if it is not already done). As patt2k has said, you can get the WAN connection switched over later.
There's a chance the tech will locate the ONT in a different place than you thought it might be. You'll want to verify at the beginning of the install where he will place it. While he's running the fiber, etc., you can run the cat5, then follow suggestions above.
Or you may get a tech who is sympathetic to your quest for low pings. He may run and connect the cat5 for you. But your best bet is to have it in place when he's ready to go live on the router.
It's the luck of the draw. You may get a guy (or gal) who digs the technology, or you may get a hookup drone who follows the book.
The book says coax WAN. For some techs, the book is mythical. Who needs a book anyway?