yes get it balanced. my buddies 351 is not balanced and he has a funny vibe at about 4K till about 4500 or so, then it smooths back out. mine is balanced and it is smooth all the way up to 6000(highest i have taken it.) it is worth the $150 or so to get it all done.
I don't the the stock rods/pistons are balanced. If you do something like aftermarket rods/pistons it would be wise to get them balanced before you put them in.....could always use a smoother running engine.