Per MOS:LARGENUM (and various guidelines and projects), most large numbers reported to unreasonable precision should be rounded. While a source might say a film grossed $75,763,102, this figure is unlikely to be accurate. Surely there were some unrecorded cash ticket sales, sales attributed to the wrong film, etc. Additionally, the figure is unwieldy; people generally want to know if it was $75 million, $20 million or $250 million. For Wikipedia's audience, the difference between $75,763,102 and $75,764,063 is trivial.
Rounding creates a more convenient -- though less accurate -- number. So, for example, Wikipedia reports that Star Wars grossed $503.1 million though the source says $503,075,849.
"Rounding" in everyday usage can reflect a number of differing practices. How much were those new shoes? The shelf price was $148.95. If you said "$148.95", you were being exact. If you said "$150", you were rounding. If you said "$148", you were truncating. Heck, some people might say "$140", which is really rough.
Truncating is simply dropping the digits beyond the point you are reporting. It is simple, but far less accurate. Rounding is a bit more difficult, but more accurate: $148.95 is, after all, much closer to $150 than $149. $140 is getting further away.
Basically, that's what rounding is: Saying which of two numbers the "real" number is closest to. To the nearest $100, $148.95 is roughly $100 (it isn't closer to $200). To the nearest $10, $148.95 is $150. To the nearest dollar, $148.95 is $149.
To the nearest $100,000, Star Wars $503,075,849 is $503.1 million.
Precision edit
Context matters. The current population of the Earth is about 7.594 billion people. In most contexts, Wikipedia would report that as 7.6 billion.
If, however, we are comparing the current figure of 7.648 billion to an earlier figure of 7.551 billion, rounding to the nearest 0.1 billion would inaccurately make it seem the population had not changed, when it actually increased a fair amount.
Similarly, we currently round most film box office figures to the nearest $100,000. This is reasonable for most Hollywood films today. We would not, however, report a figure of $27,603 as $0.0 million. $27,600 is probably better.
When a figure rounds to a 0 after the decimal place, some editors drop the zero. Realisticly, keeping the zero is better as it indicates the precision being used. "$5 million" means the real figure was somewhere between $5,499,999 and $4,500,000, a difference of almost $1 million. "$5.0 million" indicates the real figure is $5,049,999 and $4,050,00, a difference of about $100,000.