Before Approach Phase activation speeds are computed according to predicted landing weight, after activation they are computed according to the current weight. So if you activate the approach phase really early, your speeds might me 1kt higher than they will be at landing, but the error should not be bigger than that, unless you activate APPR phase 400NM out, and the speeds will be calculated correctly at landing using actual weight anyway, as long as weights in FMGS are correct (ZFW).