Stacked stone will vary based on the manufacturer (and sometimes with a corresponding price). As an installer, we can adjust sheets and pieces (with a labor cost associated) to a certain degree to minimize the gaps (for example we would cut the sheets or even each row to stack them closer to minimize the gaps) but as a rule, an attempt to disguise those variations with grout is not a good idea. It isn't meant to be grouted. And piece meal grouting....no. Grout is all or nothing, not just certain areas. That has regret written all over it. You either pay for a higher quality product, or you pay a pro to spend the time to adjust where possible, or accept the imperfect properties of the product you selected.