That's generally a judgement call (for me), LtC. Elennsar. I gather as much information on a particular subject as I possibly can. I prefer accounts from the common soldier, but I realize their limitations. I am skeptical of commanders as they tend to be self-serving and legacy enhancing. I throw-in time-tested historians, professors and authors. Then I throw-out the obvious highballers and lowballers. Put the rest of them in a bag. Shake-it-up, and construct a consensus or average. It may not be 100% infallible, but it works for me.
When folks start throwing-out #s like: 10,000, 8,000, 13,000, etc., I'm "OK" with a 10.3k average. I know that it is a heckuva lot more than 100 and substantially less than 50k. Too much analysis can result in paralysis, it is said. I get the general picture. To me, it's not that important whether Mr. Lincoln wore boxer shorts or jockey shorts.