I think kissing after flowers did add the extra 2% while dating Denise, but I don't remember perfectly.
(A successful date would add 5% and responding positively to go inside would add another 5%). The way it used to work was that the act of giving a gift added +1 percent, and if followed by a kiss it would add another +1%.
I have searched for Definitive Edition specifics on dating mechanics but could not find any.