Not compulsory for fresh installs as you will be uploading your images
Just save media settings prior to start customization / creating posts.

• Considering that you have ‘Saved media settings’, the images you upload thereafter will get these media settings applied.
• Fine if you do not regenerate thumbnail for demo data imported, as anyhow you will be deleting our demo images / posts and creating yours.

How to regenerate thumbnails?

• Install and activate : Force Regenerate Thumbnail plugin.
• Click : Regenerate All Thumbnails

** You may use any plugin you are familiar with.

Why to regenerate thumbnails?

• In WordPress, dimensions saved in media settings do not apply to previously uploaded images.
• ‘Regenerating thumbnails’ regenerates all your previously uploaded images to use the most optimized sizes for theme.
• We ship only original size images with demo data to make it more efficient.
• Also, if you are not building your website from scratch and have existing content and images on your current website those may need to fit in new theme’s design.
• Most of our themes work well with default WordPress Media settings, in case you had those previously.
• Still you will require to regenerate thumbnails for some theme specific custom elements to look nice.
• Refer theme specific help document for recommended image settings / sizes.