Models that are lapping round the flank or rear are ignored for purposes of establishing a unit's rank bonus in close combat. Lapping round models in this way might therefore reduce your rank bonus by reducing the number of ranks fighting.
However, units which are lapping round do receive the extra bonuses for flank and rear attacks so long as the entire unit has a Unit Strength of 5 or more. This means that, in most cases, it is well worth lapping round whenever you can. Note that models lapping around an enemy unit's flanks or rear do not negate the enemy unit's rank bonus - only a new charge into the flank or rear of a unit negates its rank bonus.