Not a conflict of interest--legally at least; perhaps it is ethically or morally, but that has no legal effect--and perfectly legal. A manager or supervisor may put him/herself first and give him/herself the best shifts, prioritize time off, slack on side work, work as a server or bartender as well as manager, etc. Certainly, his or her own managers (e.g. the owners of the restaurant) could disapprove of this and take action (e.g. tell him/her not to do this, or even demote or fire him/her), but the law does not restrict this in any way.