All things equal, 2.1 gives slightly less dilution to current shareholders but the same positive "optics" as 2. Likewise, 1.9 gives slightly more equity to the VCs but a lot less prestige to the company their investing in (so arguably they get less value).
The same psychology comes into play when pricing anything else. We price things at $1.99 rather than $2.00 because there's an irrational feeling that a price starting with a 1 is much less than a price starting with a 2.
I think the OP is just making the point that a lot of these $1B companies are priced as such because it gives some extra legitimacy, thereby increasing their chance of success and increasing the value of the VC's share. The question is whether this is speculation or whether investing at that price actually makes the company worth that price.
