When I make a bow, I have a particular weight and draw im shooting for. If I build a bow for a person drawing 27", ill tiller it to 27" and the weight they want. As mentioned, you will gain or lose about 2-4lbs of draw beyond or under marked weight. I would also caution drawing beyond marked draw lenght. Drawing less is no big deal, but drawing over the bowyers marked lenght can cause damage. Especially with all organic bows. Glass bows and lam bows can handle longer draws to some extent, but id still suggest not drawing beyond.

If you are having a bow made for you, go with your draw length, that way the bowyer will make you just what you want. As for your question as to why 28" is almost the standard on manufactured bows? It has to do with the fact that a majority of shooters draw 28" or less. 28" is simply a way to mass produce to the majority. That would be the only advantage to making bows at a set draw length.