O Código Elétrico Nacional diz que o receptáculo deve estar dentro do 3 'da borda externa da bacia, mas não mais do que o 12 "abaixo da parte superior da bacia (210.52 (D)). Ele também afirma que o receptáculo pode ser instalado no bancada, mas não deve estar na posição voltada para cima (406.5 (E)).
Obviamente, os receptáculos devem estar protegidos por GFCI, pois estão em um banheiro (210.8 (A) (1)).
Veja também esta resposta de @Shirlock Homes.
National Electrical Code 2014
Chapter 2 Wiring and Protection
Article 210 Branch Circuits
210.52 Dwelling Unit Receptacle Outlets.
(D) Bathrooms. In dwelling units, at least one receptacle
outlet shall be installed in bathrooms within 900 mm (3 ft)
of the outside edge of each basin. The receptacle outlet
shall be located on a wall or partition that is adjacent to the
basin or basin countertop, located on the countertop, or
installed on the side or face of the basin cabinet. In no case
shall the receptacle be located more than 300 mm (12 in.)
below the top of the basin. Receptacle outlet assemblies
listed for the application shall be permitted to be installed
in the countertop.
210.8 Ground-Fault Circuit-Interrupter Protection for
Personnel. Ground-fault circuit-interrupter protection for
personnel shall be provided as required in 210.8(A) through
(C). The ground-fault circuit-interrupter shall be installed in
a readily accessible location.
(A) Dwelling Units. All 125-volt, single-phase, 15- and
20-ampere receptacles installed in the locations specified in
210.8(A)(1) through (10) shall have ground-fault circuitinterrupter
protection for personnel.
(1) Bathrooms
210.11 Branch Circuits Required.
C) Dwelling Units.
(3) Bathroom Branch Circuits. In addition to the number
of branch circuits required by other parts of this section, at
least one 20-ampere branch circuit shall be provided to
supply bathroom receptacle outlet(s). Such circuits shall
have no other outlets.
Exception: Where the 20-ampere circuit supplies a single
bathroom, outlets for other equipment within the same bathroom shall be permitted to be supplied in accordance
with 210.23(A)(1) and (A)(2).
Chapter 4 Equipment for General Use
Article 406 Receptacles, Cord Connectors, and Attachment Plugs (Caps)
406.5 Receptacle Mounting.
(E) Receptacles in Countertops and Similar Work Surfaces.
Receptacles shall not be installed in a face-up position
in countertops or similar work surfaces.
NOTAS:
Como o @ArchonOSX aponta, deve haver pelo menos um circuito 120-Volt 20-ampere fornecendo ao circuito de derivação de pequenos eletrodomésticos do banheiro. Este circuito não pode ter outras tomadas, a menos que ele forneça apenas equipamentos em um único banheiro e quaisquer outras tomadas estejam de acordo com 210.23 (A) (1) e (A) (2). (Consulte 210.11 (C) (3) acima)