Interesting that the BigLar originally asked about the ETHICS of the scenarios, and most of subsequent posts are about the COSTS of the scenarios. Ethics should really have nothing to do with whether it is financially advantageous to do so.
As far as the ETHICS of the scenarios --
I have to go with the poster who commented about cash advances 'hidden' as a charge. That is clearly against most CC rules of usage. IMHO, the ethics tips from OK to not-so-OK (and to a resounding NO for #6

)when the magnitude and intention become substantial. When dining, I often take my friends' cash and charge the meal, but usually more for convenience or I am short on cash (I confess, the miles aspect lurks in the background). But to do it routinely and for 'large' amounts of money (not sure how much that is), then the ethics get iffy.
So, IMHO -- 1&2 seem innocent enough, probably 3 too (an isolated case); but 4&5 are a pattern of deception and, to me, are ethically deficient.
And finally -- dude, where are you buying your widgets? I get mine for $3K tops!
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Da DOK