I would say that no "good practices" for file extensions exist, strictly on a technicality: Unix/Linux/*BSD file systems don't support extensions per se. What you are calling an extension is merely a suffix of a single file name. That's different than the VM/CMS, VMS, MS-DOS and Windows file systems and OSes where a special spot in the inode-moral-equivalent is reserved for an extension.
That little rant now over, I think it's a bit silly to put a ".sh" or ".ksh" or ".bash" suffix on a shell script file name. A program is a program: no benefit exists in distinguishing what gets executed. No unix or linux or whatever kernel has decided to call an interpreter on some file just because of a file name suffix. It's all done by the
#! line, or some other "magic number" sequence of bytes at the beginning of the file. In fact, deciding what to execute based on a file name "extension" is one of the factors that makes Windows a malware magnet. Look at how many Windows malware scams involve a file named "something.jpg.exe" - by default newer Windows don't show the ".exe" extension, and encourage a user to just double click on the "image". Instead of an image view running, the malware runs.
What you might think of as a straight-ahead command is often a shell script anyway. Sometimes
cc has been a sh-script,
firefox is an sh-script,
startx is an sh-script. I don't believe there's a cognitive or organizational benefit to marking a script with a ".sh" suffix.