The scaling calculator in beersmith seems reasonably reliable within 5-10 gallons as long as you do a good job setting up your equipment profile for whatever your small batch system is. You will probably need to tweak the boil off percentage the most, especially if you are using a pot that gives you more surface than depth in the kettle volume due to using too large of a pot. However, you'll figure that out after your first time or two.
The biggest problem with beersmith IMO is that the calculations round too much and the variance created by all the rounding grows to an unacceptable level when you scale from a large commercial volume down to homebrew numbers. Going from one gallon to ten gallons might give you an error of a couple ounces but fifteen BBL to five gallons turns into a half gallon of error. Add that beersmith is not designed to account for the changes in efficiency in mash conversion and hop utilization between commercial volumes and homebrew volumes and cutting 15 BBL to 5 gallons is unreliable. If you're just cutting a five gallon batch to two or three gallons then this is not an issue for you.