The OG, FG, ABV and Color are very easy to verify by hand (and a lot of people have checked the numbers and equations I use here) - I'm very confident that BeerSmith has the right numbers here. You can add up the contributions of the various grains in points, multiply by your batch efficiency and divide by the total volume to get a very accurate OG reading. Apply the average yeast attenuation to get the FG, and the ABV follows from OG and FG. The only variable here would be what the actual potentials/yields are for the various grains you are using.
Color is only a bit more tricky - I use the Morey equation which is easy to look up online - which basically adds up the color contributions of the various grains and then applies an exponential equation to smooth it out for higher color numbers.
The IBU number is the only one that does have quite a bit of variation. If you change the hop alphas, equipment settings, leaf vs pellet, boil volumes, or even the equation you use it can have a big impact on the final numbers.