Absolutely---we are not under the law, but when God puts His finger on something in us and tells a person He wants a tenth earmarked for Himself, we ought to obey Him. There are still promises H has that are attached to the tithe that He desires we receive.
We cannot stand and tell everyone that they must do it---nor that everyone must not do it, precisely because this is not a law for us. It is a spiritually discerned matter between God and each believer. According to the New Testament, though, should God speak to an area of one's life and the person does not heed Him, then it is sin for him.