Sometimes, transient expression using PEI system needs filler DNA or Boost DNA, but I can not find any paper describing transient expression using lipofectamine system needs filler DNA or Boost DNA.
I regularly do transfections with L2K and L3K without any filler DNA, and there are no issues.
The only time I add pUC19 as a filler DNA is if I'm doing two different transfections and one plasmid is much larger than the other one. Then I use pUC19 so that the total amount of transfected DNA is identical in both conditions.