I think the point of sliding from top to bottom is to prevent people from doing the very natural (if you've played any Tetris at all) maneuver of sliding the piece laterally at the last minute.
Initially we thought this rule was to prevent "testing", but this is a good interpretation as well. My group does not allow testing, and it definitely causes occasional disastrous results. We've all had our turns being bitten by it, and it makes the game more interesting for us.
As long as everyone at the table is playing the same, it doesn't matter.