I need to put my new mailbox/post up and was wondering if I can just dig or if I need to call?

Call.  Its free and usually pretty quick.  Plus it absolves you of any damage to ANY lines - with the added bonus of keeping you alive.  

thats what I figured but I didnt know if it was a standard thing not to have to worry if its in next to the curb.

Yep, call.  The added bonus is that you will know where all the burried stuff is!  If you're a nerd like me you'll bring up AutoCAD and draw the utilities on a scale drawing of your lot / house too...  don't forget to put your mailbox on the drawing!

Definitely call, it usually takes three business days (here anyway). It keeps you happy (not having to pay for utility repair), your neighbors happy (you didn't make anyone lose power), and the utilities happy (no one had to come out of fix a screw up). Usually you have to give utility lines 16-18" of clearance on either side of markings, but your state may vary.

Anyways, dig careful regardless of where lines are marked, I locate for a phone company and I've missed some stuff in the past.

Is it ever safe to assume the spot next to another mailbox is safe? I have to place my box accross the street in front of another new house but there are already mailboxes for all but ours.

Can I have the utility company do that side for the box and my side just to let me know for future reference?