Startup scripts can be added to /etc/init.d/.

I see some with a .sh extension, and others with no extension.

Does it matter? If so, why one over the other? If not, which is considered more proper?

Also, update-rc.d allows me to add and remove them to init. Is there a way to display those which have been added, or should I just use ls /etc/rc*.d?

Thanks

PS. Relates to a Raspberry Pi

Extensions are a human-readable mnemonic. Code should never depend on the presence or absence of an extension. It makes gzip a pain to use. Use chkconfig --list to show the configuration of files in /etc/init.d/

Is the best practice not to use them, or to use them, or to use them just when you want to make it more obvious to a human?

Also, chkconfig doesn't exist. I started with Centos which I did use, but with debian, I thought update-rc was the tool.

init.d is one of the "special" folders that should only hold one type of file (a script) or a link to that kind of file. Nothing in that folder should require an extension, but I would not obsess over it one way or another.

chkconfig is somewhat specific to the Red Hat family of distributions. Moving between a dozen distributions, I use the tools, but often look for myself. That is easy on a system that uses the traditional tools, but I have not fully adjusted to the systemd world just yet and still need the tools on those.

what version of debian are you using? if it is an up-to-date version then it will be using systemd rather than sysvinit which maybe a consideration on how to implement things.

Raspbian Jessie which is a debian derivative.

Not sure where that version sits in the scheme of things. According to distrowatch the latest version (2017-04-10) is using systemd, so you would be looking to run systemctl --list-units.
The previous listed version (2015-02-16) is still sysvinit. Of course if it is anything like Oracle linux, they decided to mix the 2 together (sheesh)