Lot's of questions? *cracks knuckles*
What I want to know is when the parent themes are updated, should you also update the child theme, too.
That depends on what you've changed in your child theme.
If it's just CSS in your child theme's
style.css, then odds are you'll never have to worry about it. CSS is cool like that. ;)
If you've made copies of the parent theme files such as
sidebar.php, then when an update occurs to the parent theme, you should check your copy against the parent copy. That way if it's really a critical update, you'll be fine also.
Please be aware that if you're a little clever then you don't need to modify parent theme file copies. With CSS,
add_filter() in a child theme
functions.php file there's often a way to make your changes without editing any copies of the parent theme.
Also, in bluehost it was recommended
That's a good host and their mostly right. :) disabling plugins is not a hard requirement but it can help make for a smooth upgrade.
I've never had to deactivate plugins or switch themes for any of my upgrades. But I keep really good backups and can restore the whole magilla to just last night if I have to.
Unless your WordPress installation is doing something goofy (using a poorly coded or dated plugin or theme), upgrading to the latest and greatest is not a bad idea. And deactivating your plugins and switching to the Twenty Eleven theme can make that a very smooth upgrade.
BUT even with that, backup your files and database and educate yourself on how to restore both. Sometimes unintended Bad Things™ happen and good off server backups are a great safety net.
That way even if the upgrade is a complete disaster you can put it all back the way it was.
Does one HAVE to upgrade?
It's recommended but it's really up to you. It's easier for us other volunteers to provide support when someone with a problem is running a current version.