+1 on leaving it blank if you aren't getting distant traffic you don't want to hear. Sometimes part of the transmission gets chopped off because the scanner doesn't recognize the CTCSS tone fast enough. Also, these scanners sometimes have an issue with CTCSS and pager tones. CTCSS is supposed to be cut out when pager tones are dropped but some agencies don't have it set up that way; I have a couple in my area. And sometimes the blend of tones will cause the scanner to drop the channel and resume scanning, causing you to miss part of the transmission before it re-acquires. Leaving CTCSS blank for those frequencies eliminates this problem.