how much sag is too much

You probably should use the WDH since you have it. It will help put some weight back on the front. Do you need it? That's hard to say without some actual weights. If you're tongue weight doesn't exceed you hitches weight carrying rating and your not exceeding your rear axle rating then you don't actually need to distribute the tongue weight.
I tow 3 different trailers from 4,500 to 16,000 pounds with no WDH. The light weight one is a boat so only has 216 pounds of tongue weight. The heavy trailer is a flatbed so I can load it properly to manage the tongue weight.