Reach early and do your testing first before most of the guest arrive, the testing should not take you more than a few minutes.
In most hotels at least 28mm (full frame) @ f5 is needed due to the space available between tables and a flash is a must. Zoom lens will be a better choice as there may be shots (during the table shots) where the guest wanna drink with the couple and closer shots will be needed.
For amb. light in the background, my personal preference is about 1 stop under compare to the flash, anything more that that may have a colour cast on the people in the shot. This is also a "safety" as the lights are not constant throughout the whole ballroom ie: if the couple are standing under a downlight/sportlight shining down on them, they will have a colour cast on them compared to the rest of the guest.
At the end of the day as many had said, get the shot right first. In table shots speed and how you handle the guest is important.