I'd only lip-synch to a studio-recorded song if it was ironic or fitting for my character to do so. For example, as Light I could do a skit that re-imagines the end of Light's reign as kira with him getting away, ryuk telling him in person that he will keep his promise to take his life when it's over, and in his last three minutes Light could sing Dust in the Wind by Kansas. (To signify that how in the end his attempt to change the world was futile.)
Even so, I'd tell most folks it's okay to use the original song since few people can sing stage-quality stuff.
Tetris DX top score - 02,323,878 points/1501 lines
Tetris DS top score - 11,014,897 points
Anime Mid-Atlantic is the next beast! Choose your cosplay now!
Coming soon - my most patriotic costume yet, Captain America!
Cosplays on the Drawing board -