Is this a requirement? I have never pulled the battery and not had any problems...<knock on wood>
What are the drawbacks to not doing it?
I have noticed that if you don't reboot after installing the new theme, your memory drops like a heavy rock. Check your memory and see how much it's dropped since changing to a newly installed theme. If you normally maintain a high level of storage on your device, you probably don't need to worry about it. But, if you're hovering around 25-30% storage because you're adding themes, apps, and lots of data, it's a good idea to reboot to protect your memory from dropping too fast.
Install the theme. Activate said theme. Restart your device.
There is no point in restarting after the install since you haven't even loaded it yet. The reason for restarting after activation is to cure "theme hangover" where remnants of an old theme, such as graphics, or even scroll paths still reside in the new theme until a reboot.