It isn't really a matter of being "smart". Even investors, who have detailed financial models, decades of data for comparables, get preferences and often board representation to protect their interests, aren't confidently going to be able to provide a probable value of the investment. It is certainly unreasonable to expect employees, who get no board representation and thus can be screwed over in any scenario short of an IPO, to do better than people who have this as their entire job.