How to round properly has been studied in detail by physical scientists (you need rules so people don't distort their measurements, intentionally or accidentally), and by linguists. With respect to correctness, you can take a page from the physical sciences and think about "significant digits": If you estimate a number to an accuracy of about one hundred, it makes no sense to report it as "2738"; the final digits have no validity, so you might as well round to the nearest 50. In short: Round to steps that are about half the size of your uncertainty, and round to the nearest round quantity.
With respect to impressions, think also about what your users want to know. If they just need an indication of how many pages of results follow, "around 2700" is fine, while "around 158,270" may strike users as slightly strange. You must have noticed that google search results have a header like "About 7.840 results (0,67 seconds)", which conveys that the true number of results may be different (and will most likely change as you page through them).
When we don't have a specific need for an exact figure, an estimate to within 10% of the correct figure is good for general purposes [citation missing]. That's two significant digits, so providing many more may strike the reader as strange. Krifka (see link above) mentions the oddness of Swiss street signs that warn "stop sign in 103 meters".
Now about rounding down. "More than X" is technically true for any quantity greater than X, but as @Ayyash alluded to in a comment, it's usually strange to underestimate by too much. Old example: Imagine McDonald's advertizing "more than 1000 hamburgers sold". The technical explanation is that it's semantically correct but pragmatically anomalous, because it gives less information (that is, useful information) than the author is capable of giving.
However, a low-ball estimate is indeed appropriate in some contexts: Airlines often report "more than 6 seats available" even if the plane is empty: the exact number is irrelevant to most users, who just need to know if the flight or fare is almost sold out. (This also allows the airline to avoid giving out too much operational info.)
So in the end, what accuracy you provide depends on your circumstances.