Use the gauge. You can get a cheap one at AutoBone for like $19. Yes the 90 deg is easy to eyeball, but there are times when you may have to pick up the wrench and move it to complete the 90 deg rotation due to the hex being in the wrong location or some interference in the engine bay hitting the wrench handle, e.g. the firewall. It's poor economy to save $19 for a potentially $$$$ screwup. In Oklahoma we used to call that kind of thing "a dollar tripping over a dime", trying to save a nickel on a potentaily expensive screwup.