It would be rather contradictory to his character if Mihawk never once set foot in Wano. According to his Vivre Card, he continued to fight until there were no more challengers left. If he never visited Wano, that puts one big asterisk behind the claim he fought every possible challenger.
The reputation of the Samurai from a closed off nation should entice someone like him. Yet he never once featured in Oden's flashback. A flashback which had cameos of many named characters. Including his fellow former Warlord Moriah.
It's for that reason I think he did visit Wano. But it would have to be during the time when Oden wasn't there. Either after his death, so during the 20 years between Oden's death and the main story, or during the years when Oden was sailing with Newgate and later Roger. We know he was an active Pirate during that period. And he would've been very young at the time if it was when Oden was adventuring, thus more eager to test his mettle.