Sugar.... we're addicted to it! Even though we know it's not really all that good for us. But, really now, if we eliminated everything from our diet that someone says isn't good for us, we'd have a very boring diet. I can't think of much that someone hasn't said is bad for us. I tend to go with Mark Twain's advice, "Moderation in all things, including moderation" and "Too much of anything is bad, but too much whiskey is just enough."
There are two major reasons people add sugar to dough. One is to "wake up" or "speed up" the yeast. Some say that yeast can't digest starches. (Why am I talking about yeast on a sourdough web site? There is a LOT of yeast in a sourdough starter, and if you want your bread to rise, you want your yeast to be happy.) However, yeast can digest starch. Yeast has enzymes that breaks down starch into sugars and then eats the sugars. And since I'm a big proponent of long slow rises, I don't really want to speed up the yeast. It'll wake up when its ready, whether it's a pure bakers yeast or the yeast in a sourdough starter.
When I see instructions to feed yeast some sugar, I just ignore the instructions. The yeast will work just fine without the tablespoon or so of sugar.
The other big reason to add sugar is simpler. It is to make the baked goods sweet. We love sweet things. As you look at the recipes here at Sourdoughhome.com you'll see we don't sweeten things as much as many other people. We like sweet, but we like it restrained.
That said, when you add a little sugar to a recipe it will rise faster. A little more and it will rise even faster. And when you add still more, the yeast slows down. At what point does that happen? According to Red Star Yeast, a dough with more than 1/2 cup (64 grams) of sugar in 4 cups (520 grams) of flour is a sweet dough and will start showing the effects of too much sugar.
The sugar will compete with the yeast for water, it will also create a higher osmotic pressure which can kill yeast. You can compensate for these effects by using as much as 50% more yeast or using an osmotolerant yeast. Osmotolerant yeasts are yeasts have been selected to resist the effects of osmotic pressure. A number of yeast companies make osmotolerant yeasts, we used SAF Gold label for a number of years. A few people have created their own osmotolerant sourdough starters by feeding the starters 80% flour and 20% sugar for several days.
I tend to work hard on the notion of not asking for trouble. Try a recipe with regular yeast or starter. If it has a lot of sugar and doesn't rise, you might look for an osmotolerant yeast or create an osmotolerant sourdough starter.