I don't recall having undue problems with low weight landings and I wouldn't advocate adding extra knots. Having said that, if you have significant Vref additives due headwind component (half the HW component eg 30 knots HWC then add 15 knots to Vref) you might get into a float problem trying to get it on to the ground.
It should be realised that the HWC additives must be gradually reduced on final approach so as to cross the fence at Vref. As the free stream airflow is considered to start at around 2500 feet above ground level, and thus the wind strength gradually reduces due to ground friction below 2500 feet - the Vref additives of HWC (not gust, though) should start to bleed off from say the outer marker height (or equivalent)
There is no way you can bleed off say 15 knots of HWC additive at the flare, to touch down slightly below Vref. Most aircraft will float due non-removed additives and the aircraft may have to be forced on the runway.
Is therefore, your problem with light weight 737 landings, not due to light weight - but perhaps to excess over-the -fence speed above Vref? Just a thought.