Yeah, if they aren't contradictory to your beliefs, and are not blasphemous or irreverent, there is no reason not to sing along. We share many hymns with many denominations. My problem with 'Christian' music, such as it is on the radio and such, is that it tends to be either insipid or too irreverent for me, or both. I wouldn't sing along to such music myself. For instance, there are churches where I live that have set religious words to rock music. That can be great if done tastefully, but the other end of the spectrum trespasses on taking the Lord's name in vain.