I'd have to check to be sure, but offhand, I don't think object spacing is preserved in EPUB export. You have to use that Space Before/After Setting to get reliable results, which can be overridden in the individual image export settings.
ETA: Just tested and confirmed this. Spacing has to be set using the export values, or CSS. Object spacing set within ID is largely, if not wholly discarded. (On a technical level, I think it's because such spacing is seen as "padding," which ID does not export to EPUB.)
ETA2: As for the overall question, it's always better to use spacing-below for EPUB. While adding space above the paragraph would work (I'm pretty sure... the details are sometimes fine), it would also add space above the paragraph should it break to a next page. Space below is always "lost" by readers and doesn't interfere with an object or paragraph being at the bottom of a page. (EPUB secret #423a: a good way to force a page break in most psuedo-paginated readers is to add a large space-below value, such as 999px, to the prior paragraph. This can create a corresponding gap in linear readers... which is a minor flaw, IMHO.)