When you tie up in a slip, you have the choice to cross the bow and stern lines or to run them straight. There are advantages to crossing in certain situations, but as with everything, there are drawbacks.
The reasons to cross the lines in the slip are to provide a longer line leading from the boat to the dock cleat. This longer line will provide more stretch as well as make tidal variations less sensitive.
This sounds all well and good, but the problem that follows with crossed dock lines all pertain to chafe. The lines will lay in contact with each other as the boat moves around in the slip, causing the lines to rub against one another. If left unchecked, the lines could eventually chafe through causing the boat to drift into the pier and sustain damage.
The only time when crossed docklines are deemed mandatory are when you are tied up to a fixed pier in an area with a great tidal variation. Crossed docklines will provide the boat with the flexibility to rise and fall with the tide without threatening to rip the cleats out of your hull. Any other situation, crossing the docklines offers no advantage and only invites the issues associated with chafe.
While chafe can be prevented by supplying the lines with chafe covers located at the point of contact, it is best to avoid the problem all together if possible.
When tied to a floating pier, the pier and its cleats will rise and fall with the tide, along with your yacht. Crossed docklines will serve no advantage here.
If you need the additional length of line to help absorb shocks as your vessel moves about in the slip, the correct solution should not be to cross the docklines, but instead to select a dockline that offers more elasticity. I see people with all sorts of contraptions to offer elasticity in their double braid docklines, when the truth is they should use stretchier Three Strand Nylon which will offer all the stretch they would need.
Next time you tie up to a pier, look at the situation before you cross your docklines. If you need to, then do it. If you don't actually need to, avoid crossing your lines and avoid all the problems of chafe that are associated with it.