Fuel leaks

Are you talking about the fuel return line that is connected to the bottom of the regulator or the supply line connected to the rail? Either way, if you are talking about one of the threaded lines then yes you can tighten them but I would take it off and replace the o-ring and then tighten them. If you over tighten the line then you will need to replace either the rail or the regulator because you will crack it or strip the threads off the line. Plus they are probably leaking because the o-ring failed and not because they are loose.