When you boil the wort, you drive much of the dissolved gases out of it. Yeast need dissolved O2 to reproduce during the early stage (aerobic) of fermentation. Dry yeast is packaged with sterols which provides the O2 needed to get them through that stage, but not the case with liquid yeast so extra aeration/oxygenation of the wort is critical to yeast health and a good fermentation.
Splashing into the fermenter is fine if you're using dry yeast and that's what I'll do even though I have an O2 tank and stone. With liquid yeast (especially in a lager ferment) however, you really ought to make the extra effort to introduce new O2 into the wort. Pure O2 through a 0.5 micron stone gives the highest level of dissolved O2, but it's not cheap to get set up that way. Next best is the aquarium pump/stone or aeration wand on an electric drill. Some folks will shake the fermenter, but 5+ gallons is a wee bit heavy.
It's also important to aerate your starter wort.