Now, I view this more from the point of the first responder. I could give a care about the "optics" of a situation. I think that President Bush was 100% right in his call to not go on the ground too soon after Katrina. I think President Obama hit just the right time in Alabama, after the majority, if not all, of rescue and recovery was completed, but before too much clean up had happened. He could do all of it: be a consoling presence, show leadership and get the really cool pictures in the rubble, without being disruptive to what was going on.
So here are my parameters: The visit must not complicate the lives of the fire service, EMS, or law enforcement at all. The president should be as "self contained" as possible--bringing as much of his own logistics as he can. Saving of life and property must always be the first priority; if a president's visit will slow that, it needs to wait. There are other ways to show that you care--videos, radio addresses, statements in speeches, etc. etc.--that can be used until a ground presence is not a major hindrance. People do appreciate the president coming by, I think that is true, but it must not take resources from the immediate task at hand.
We in Missouri will welcome Mr. Obama when he comes--currently scheduled for Sunday. It might be sooner if conditions in Missouri can tolerate it and the president has to return from Europe early due to the volcanic ash cloud's affects on air travel.
Addendum: I left this post and I was pretty satisfied with it. And then I wasn't. So I am adding this and modifying the last paragraph, which had suffered from an invasion of partisan snark. Because I am not going to pick at any president politically for how he handles going into disaster sites. Or not going, as the case may be. People who pick at this topic are incredibly inconsistent: Republicans defend Bush for not rushing to New Orleans, yet rake Obama over the coals for not going to Missouri. I am thinking you are better off sticking to general principles, and judging according to those principles, applying criticism evenly, regardless of political labels. Using disasters for political gain is incredibly distasteful to me. It is not a time to trot out nasty partisan politics--it is a time to pull together and help those who need help, both now and in the long term.