Just bolt it on. There is almost no play in the mounting bolts in the frame rail, so a change in alignment specs is unlikly. An alignment will be your choise, if you think it is out, then yes. If the alignment was ok before the idler went, then it should be close again. If you are not sure, look at you tires....$60 for an alignment, $120 apiece for tires....
BTW if you get an alignment, the alignment tech can tell you if any other ft end components are loose.