Is it just as difficult to find a desired output in a hash digest, for MSB vs LSB?
As the other answers point out, output bits of a good hash function are uniformly distributed, so your substring has equal chance to appear in any part of the hash digest.

However, in the case of Bitcoin, the address is not a random string. Not only it starts with a predefined number (1 or 3), but the string is the result of the Base58-conversion, which may (I did not check) produce biased outputs. The addresses may even be of different lengths.