In general, OmniFocus does not give you the tools you need to create a "today" perspective that doesn't either include some unwanted items, or exclude some that should be present, or both. Therefore, it is counter-productive to try to Nash that square peg into the round hole. Instead, make yourself a set of perspectives that give you the desired set of tasks, and rotate between them repeatedly as you work. I think it is safe to say that everyone wants to complete the tasks due today. Make a perspective that shows them. Use that perspective to clear them out as early in the day as possible. Use another perspective to grab the tasks which are most important, whether you use start dates, context, flags, or whatever to indicate that. Work those tasks after the immediately due tasks. If you weren't able to completely clear the due today list, check it occasionally throughout the day as you encounter different contexts, start dates/times tick by, 3rd parties respond, and any other gating factors are removed. If you're extremely productive and manage to clear both of the lists, look at upcoming due items. Check the overall list of available items by context. Depending on your method for utilizing context, doing so might be appropriate higher up in the list — I make some very narrowly defined contexts to cover tasks which I want to handle whenever the situation permits, but aren't always sufficiently important to merit a trip to that context. If I happen to find myself in that infrequently-visited context, I want to knock off any tasks there even if they aren't otherwise near the top of my usual lists, so I'm not forced to make another trip at some later date (which is probably more disruptive to my overall productivity than spending a little time now). I'm not likely to make a special trip to install some piece of software on my brother-in-law's computer (it can wait until the next visit), but if I do visit, I'll want to get all of those tasks done, temporarily elevating their urgency. That urgency evaporates as soon as I leave, so I prefer not to indicate it with start/due dates and flags.
The point is not to convince you that my method of selecting tasks should be yours as well, but rather to dissuade you from trying to find a Grand Unified Perspective. Switching between perspectives is easy (you can even give them toolbar buttons and keyboard shortcuts), so you should use as many as you need to find exactly the right tasks rather than restrict yourself to one which does a poor job.
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