OSHA does not actually specify when foot proction is required. They leave it up to the employer to make that determination. However, if you have injuries due to heavy things falling on feet you may have a hard time justifying your decison.
Excerpt of a letter from posted on OSHA's website:
The OSHA regulations pertaining to employee footwear are found at 29 CFR 1910.132 and 1910.136. In general, the standard requires that foot protection be used whenever it is necessary by reason of hazard of processes or environment which could cause foot injury. Normally, the employer will determine which, if any, of the employees are exposed to a foot injury hazard. This does not mean that those employees requiring foot protection are required to wear foot protection at all times when working. When employees are exposed to the possibility of foot injuries, foot protection shall be worn. When employees are not exposed to possible foot injuries, foot protection is not required by the OSHA standard, which then becomes solely a matter of employment conditions existing between the employer and the employees, and where applicable, subject to any labor/management contractual agreement.