When you say recipes call for grains you don't have on hand, do you mean for a different brand such as Breiss versus Great Western, or perhaps the Lovibond rating for a roasted grain is 30 points different from the one in your inventory?
As long as you can get close to the type of grain, the PPG, and reasonably close to Lovibond ratings it shouldn't make a dramatic difference in the finished product --especially if it is a new recipe and you don't know what to expect.
Why? How you perceive the taste something is a product of your genes and your experience and is individual. I used to occasionally read a brewing magazine that would have a panel of four experts taste, review, and write commentary on a different commercial beer each month.
That commentary was often quite different in the details of what each experienced.
If on the other hand it calls for acidulated malt to reduce the pH, and you don't have that or use another method to reduce the pH, then you have a functional difference and the result will taste quite different.
As an example:
I brewed for a number of years using single infusion BIAB. Then I got married, moved into a condo with less square feet, sold or gave away my equipment and gave up the noble craft of brewing.
Two years pass. Then the son of one my late friends in my local homebrewing club offered to brew my own recipe for my favorite porter. I used to get most of my grains in from a local microbrewery that has since closed.
I ordered grain from a commercial supplier and most were different brands. I brewed BIAB, my friend does fly sparge without any bag.
Different brewer, different method, different brands of grain and minor substitutions.
The results: same good beer as before.
Identical. Probably not, although I don't have a sample of the old.
Proper sanitation, good control of temperature during fermentation, allowing fermentation to finish, and generally good brewing methods are much more important than whether you used brand x or brand y.
Happy brewing and tasting.
