There's no point in a 90degree turn costing anything. It's absurdly easy and fast to do.

Go watch some sparring at a weapons class of some kind. While you're going to try to keep facing a single opponent, when there's a group doing 1 vs 2 or 3, you can pivot 90degrees in a split second. It's literally a half of a step at most. It's more repositioning your feet than moving any distance, which you're gone to be doing anyway; you don't fight flat footed, you use footwork.

Even in armor, fighters stay mobile and shift positions in a split second. You can even pull a 180 with one long movement of a leg, then a short one of the other if you practice enough.

Now, this isn't to say the there aren't drawback. It might make attacks of opportunity available, if there's multiple opponents.

There's no real issues with covering the an arc from each side across your front at all. So, the three in front and the two to the sides shouldn't have any difference in advantage at all either. The only time advantage would come into play is vs multiple attackers, which already gives you disadvantage.

Seriously, I'm damn near fifty and disabled. I can still pivot 90 degrees in no appreciable time. Once you've spent any time training or actually fighting, it just isn't a factor.