Depends on your relationship with the elder. If it is someone you are not on "friends" terms with, stay formal. BTW, don't be too fussy about this stuff. The Japanese are not as tight-assed as some people would have us believe. If you use a wrong form, you won't be chastised!

I often use "ja nai yoo" to my parents, teacher, my grand parents because they are close people with me. But I never use "ja nai yo" to pople I meet first time, and teachers I don't speak much. when you make good relationship between you and elder person, you can use "ja nai yo" It is casual phrase, so you can use it to your friend and your boy friend, and your family. I'm sure that you shouldn't use it to your boss of your company(^_^;) please tell me, there are mistakes in my English!