Theres is no mention of this vref+8 kts app speed in any manual.
Even for older classics with bent wings(ie not that smooth) an increase of app speed is not mentionned as far as i know.
It is up to you as a PF to follow boeing procedures(fctm).
If your training captain or TRI teaches you to add these few knots,just do it for CRM...once you will be captain you will decide and act by your own judgement.
It doesnt hurt to ask the person in question why he does so(for all other issues too),then you will decide for your own.
Obviously the FCTM is clear.
In real world,(ie not sim),you will see that most pilots arrive into the QRH flare area of 50 ft at app speed(vref+5 mini or vref + gust) as being at vref exactly at 50 ft needs quite a good skill and any change of wind or retarding the levers just bit too quickly may get you below vref.
So some accept a 5 kt extra speed knowing their landing distance based on a/b setting/weight/rwy condition.....
If you are close to your max ldg distance available then id make sure to be at vref.
Not being below vref is important as it is your minnimum speed for flaps retraction.
At the end of the day we all try to be as precise as possible but when theres room,one may accept a comfort margin...
Good day.