Do you need to wire a commercial building with wooden studs in conduit or can you use romex?

According to the 2008 National Electrical Code book (US NEC):

Section 334.10 Uses Permitted. Type NM (NM is the proper term for the brand name product Romex) cables can be used in:

(1) One- and two-family dwellings.

(2) Multifamily dwellings when built as a Type III, IV, or V construction

(3) Other structures when built as Types III, IV, and V

(4) Cable trays in Type III, IV, or V structures

[Types of building construction and occupancy classification are defined in NFPA 220-2006, Standard on Types of Building Construction]

So yes, Romex can be used in a commercial building. There are other code sections that may apply to your particular installation, if you are unfamiliar with the code book and are doing electrical work you should take a class on the NEC. An improper installation could lead to personal injury or property damage. Electrical work should only be done by a qualified person. The NEC defines a qualified person as "One who has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training to recognize and avoid the hazards involved."