If you have gone to the trouble of having your race course certified according to USATF standards, it is important to hire a company that will time your race in accordance with those same standards. If you hire a company other than GSRS then this may not be the case. There are strict procedures that must be followed for the USATF to accept times as valid and certified. Most timing companies don't bother with these details. In many cases the times they supply will be correct or close to it, but without following the proper procedures, they cannot be certified as accurate.
When you hire GSRS, you can rest assured that your race will be timed and scored in accordance with the USATF standards as described in their "Application for the Recognition of Road Race Performances". We will fill out the necessary forms to certify any records that may occur at your race.
For more information, contact the USATF or visit their website. Specific rules for timing are detailed in their official competition rulebook, sections 128 and 165.