Examples of sliding
This is an interesting dilemma. The first is fine--if you happen to get another L- or J-piece in time. The second one forces you to depend on the J-piece instead of either J or L. However, the second is not a bad choice since it allows you to connect the 7th and 8th columns to the left side of the field. This gives you additional options to stack over.
Below is a very useful tactic, pioneered by player Integration. It is more useful to have nine connected columns compared to eight. If you create a 4x1 indention next to the Tetrising column, you will always be able to fill in that I-piece later. In the meanwhile, by stacking over it, you gain more options for placing pieces.
This possibility can affect decisions such as below, which may otherwise seem evenly good.