Choosing Between 9' vs 18'
If your CB installation is done perfectly, it shouldn't really matter what length coax you use. But we've found that in the event a less-than-perfect antenna location is chosen, coax lengths of 18' help to lessen the problem vs. exacerbate it. For this reason, we recommend 18' coax for mobile installs, as it provides a bit of performance cushion in case the antenna isn't working at 100%.
That being said, using a 9' coax isn't a problem. Just understand that 9' lengths will provide less "grace" in the event of SWR issues / a non-ideal mounting location, so you'll want to take extra care to install your equipment properly and ensure a properly tuned antenna.
25'+ for Base Installs
Coax in lengths of 25' increments are commonly used in permanent / base installs without issue.