Sign out from an iphone after transfer data to new iphone

Yes it's safe to wipe it. Make sure everything transfered to your new phone before you wipe it clean.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Singing out after the complete transfer is fine.

Before you sell, give away or trade in your device, you should first transfer information to your new device, then, if applicable, ...

What to do before you sell, give away or trade in your ...