Generally, yes that's the case since most iCloud items are pulled into the device as-needed (e.g. iCloud Drive and iCloud Photo Library) and iOS removes them from the iPhone if/when needed.
But depending on how you manage your photos, 64GB can be too small. If you enable iCloud Photo Library then you should be OK since photos can be automatically managed between iCloud and iPhone storage ... just be sure to choose "Optimize Storage" in your settings for the Photos app to allow this automatic management. Where photos become a problem is if you don't enable iCloud Photo Library, then you have a "Camera Roll" and photos remain on the iPhone (and upload to iCloud Photo Stream, depending on your settings). Having a Camera Roll on your iPhone means having to manage the contents yourself (i.e. save to your computer and then delete from iPhone).
But the general rule of thumb still holds (IMO) ... buy the most storage that you can reasonably afford.