Polio was eradicated from the USA in 1979 and from the Western Hemisphere in 1991. Worldwide, polio remains endemic in Nigeria, two states in India (Uttar Pradesh and Bihar), Pakistan, and Afghanistan. (see Polio Eradication – Infected Countries)
According to the CDC, between 1980 and 1999, there were 152 confirmed cases of paralytic poliomyelitis in the US. Significantly, 144 of these cases were actually caused by the oral polio vaccine (OPV). OPV was subsequently replaced by an inactivated polio vaccine (IPV) in the USA in order to avoid paralytic poliomyletis caused by OPV vaccines. 
Up to 95% of all polio infections are asymptomatic. Estimates of the ratio of asymptomatic to paralytic illness vary from 50:1 to 1,000:1 (usually 200:1, or 0.5%). Many persons with paralytic poliomyelitis recover completely and, in most, muscle function returns to some degree. Weakness or paralysis still present 12 months after onset is usually permanent. Generally, 2-5% of paralytic polio cases amongst children result in death. 
The IPV is often included in combination vaccines which contain aluminum adjuvant, a known neurotoxin (see Does Aluminum cause vaccine-injury?), but is also available as an individual vaccine (Sanofi-Pasteur IPOL vaccine) that does not contain aluminum adjuvant. Parents in the USA might consider whether the individual polio vaccine should be used rather than using a combination vaccine that includes polio.
If a parent is considering whether to modify the timing of the vaccine, consideration should be given to the risk of being infected by someone flying from a country where wild polio is still transmitted (see Disease Risk – Polio) versus a potential risk of vaccine-injury.
 Centers for Disease Control and Prevention: Epidemiology and Prevention of Vaccine-Preventable Disease. Atkinson W, Wolfe S, Hamborsky J, McIntyre L. eds. 11th edition. Washington D.C.: Public Health Foundation, 2009.