It doesn't matter (and for some curves— e.g. ones where the x^2 term is non-zero, though IIRC in scep256k1 there isn't a tidy LSB pattern, some 32 bit LSB patterns are unused entirely). About half of the X values are not points on the curve, but this is accounted for in the order of the group. There are ORDER points on the curve, and the private keys 1..ORDER-1 uniquely map to them.  Lets say that all the X values were even— they're not— but lets say— it doesn't matter since any search is already limiting itself to valid X values, e.g. any statement about the security already excludes the points which are not part of the curve, which can't be reached by any private key, and which wouldn't be included in any key search.