In support of gulliBell's statement with a slight correction.
On a correctly marked heliport (i.e. in accordance with ICAO Annex 14 Part II):
Having the pilot on the touchdown/positioning marker (i.e. the (yellow) touchdown positioning circle (TDPC) or other 'defined' marking) serves two purposes. It ensures that:
1. the undercarriage is within the load bearing area (TLOF); and,
2. all parts of the helicopter are clear of obstacles (FATO).
That does not necessarily mean the wheels/skids are within the TDPC – the minimum size of the TLOF plus the attributes of the TD/PM are designed to ensure that element 1. is achieved.
Further; the obstacle limitations ensure that the undercarriage is protected within the TLOF and other moving parts are protected within the FATO.
Not being correctly positioned negates these assurances.
Jim