OP, if you haven't bought the closeup photography books by Lester Lefkowitz and H. Lou Gibson mentioned in "the list" please get them and study them. Gibson makes the point in text and with example photographs that stopping down can reduce DoF. I've seen this myself at high magnifications. You're probably not going to work at magnifications high enough for this to be a bad problem, but you should be aware of it and of the ways in which the rules of thumb we rely on when shooting at normal distances fail close up.
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