[contravariant]

Not any string, but if it is a multiple of 4 characters then yes it will always be valid. In particular 'changeme' has 8 characters and therefore represents exactly 6 bytes.

It gets more complicated if you're using base64 to represent a number of bytes that isn't a multiple of 3, those would be unlikely to happen randomly. Those will usually end in a number of '=' signs to pad their length to a multiple of 4 and indicate how many bytes are missing. Although apparently there are also versions o base64 that don't include the padding.