I think it depends on what you've got. If you have some mix of grains that will play nice together, frank it up. If you've got a bunch of weird ass odds and ends, maybe save them out. For yeast, I would say see if you end up close to a certain style. If you have the makings for a stout/porter, for example, use the proper yeast. If you want a good general yeast for a "whatever I end up with" style, just use a basic all-purpose dry yeast like 04, 05, or Nottingham.