The post boil volume is directly related to your OG. A larger volume means a lower OG and visa versa, so accurate measurements are important. When you're steeping grains they absorb water, taking away from the starting volume. You still have boiloff and losses to trub, unless you dump everything from your kettle into the fermenter. If you leave anything in the kettle, remember that you're also leaving extract there, so you can't expect to top up to your batch size and have the predicted OG unless you show the loss in your equipment profile. So, volumes definitely have an impact on OG. You also need to select the correct type of brewing, ie: extract, partial mash. BeerSmith relies on your inputs in order to come up with accurate predictions for various parameters. The more accurate your entries, the more accurate the predictions.
Your beer will be what it will be. We're talking about making the software accurately predict what it will be.