Yes and no; Slytherin and Gryffindor are bitter rivals, but can be more alike than people realize (which is dumb, because look at Harry). It's like dating someone from a rival college, in that it's hypothetically fraternizing with the enemy, but no one but the most stuck-up assholes gives a shit. I am in agreement that this (Sherlock as a Slytherin and John as a Gryffindor) is a more characteristically accurate and visually pleasing match-up.
Also, Slytherins can't stand Hufflepuffs (no matter what great "finders" they are), as we think they're often disgustingly bubbly and thick-as-bricks. At least the Gryffindors can hold an intelligent conversation and break the rules to have some fun once in a while.