It is always important, at any distance. That is why the 357 is so much more effective than the 38+P.
Effective at what? Expansion?.....bullet construction can give you a .38Spl round that expands at lower velocity. Penetration?....see above.
Well, let's look at what they do from snubs. According to Brassfetcher, a 158gr Speer Gold Dot .357 from a 2.5" barrel impacted the gel block at a velocity of 1047fps, penetrated 13.5", and expaned to 0.198 square inches (this averages to 0.502" diameter). Likewise, a 180gr Winchester Black Talon .357 Magnum from a 2 1/2" barrel impacted at 1092fps, penetrated 13.9", and expanded to 0.314 square inches (avg. diameter 0.632").
By comparison, a .38 Special with modern premium bullets has trouble meeting those numbers even when given the advantage of a full 4" barrel. The Speer 125gr .38 +P Gold Dot impacted at 888fps, penetrated 12.6", and expanded to 0.199 square inches (avg diameter 0.503"). The Winchester 130gr SXT .38 +P impacted at 923fps, penetrated only 9.8", and expanded to 0.299 square inches (avg. diameter 0.617"). The only .38 Spl loadings that could rival or exceed the .357 Magnum in both expansion and penetration were some of the Buffalo Bore offerings, but their velocities are much closer to standard .357 Magnum numbers than they are .38 +P.