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I think you have hit the nail on the head. Especially for someone who does not ye know what it is, it is difficult to say what angle is best because diagnostic characters in different families are usually in different places. Visible wing venation in some groups is a must, in others it has relatively little (additional) value. Legs with setae can be extremely helpful but sometime the legs provide very few useful characters. In some groups you want to have dorsal views of the head, in others lateral views, and still others frontal views; and even then you sometimes not a single one of these will be sufficient. Even oblique views may have to be taken from different angles if dust patterns are involved, for example on the mesonotum or the abdomen.

And then there is something else to take into account: a specialist may look at completely different characters then a 'general dipterist'. For the specialist a single seta on a leg may suffice, where-as the general dipterist may only be able to idenify it using a key when he has dorsal shots (wing, mesonotum), frontal shots (face antennae) and finally lateral shots (legs). When I tell you to take these shots in that order (dorsal, frontal, lateral), we may never be able to identify it to species because the fly had long since departed after the first dorsal shot. If the opportunity had presented itself, you might hav taken the lateral shot first...

Still, this is all my opinion. I am sure other people will have different opinions, so, just ventilate them. :)