YEs if you end up installing openSSH you should change the default "alpine" password using terminal or ISSH from the app store. type su - root then type your password as instructed then type passwd
No no need to change it, unless you are really concern about privacy, then you should really know what you are doing and so you won't even ask this question, I've never changed my password, always default.
OpenSSH is an app in cydia that allows you to access your iphones root directory through wifi, though it makes the default password :alpine: and its just safer to change that password if you ever install it
The only iPhone virus ever to affect general iPhone users was the one that used the openSSH default password to gain access to your device. If you change the password then there's no problem but you only need to do it if you have openSSH installed and turned on...