In general you ignore the speed rating. However, in this case, it barely matters. Unless you're in a LIVE league, the frequency of base stealing attempts is based on the RL SBA/opportunities. If you set Hargrave to 0, he will attempt 0 steals. If you set him to 5, he might attempt 5. Odds are if he goes 5 times he'll make it 3 or 4, so it will be slightly better or slightly worse, on average, than if he never attempted a steal. Same goes for basically any other steal setting.
A better example might be Paul Molitor. '93 to '95 Paul Molitor stole enough bases to matter at least a little bit, and in each season he had a very high success rate. His speed ratings for these seasons are all in the 50s. Ignore them. '94 Molitor was 20/20 in stolen base attempts. I'd probably have his stolen base setting at 5 regardless of the opposing team and catcher.