We are in the process of the same remodel - Had the same question. I got this info from the mantel company we are considering - in your case , build the mantel for a dry fit, remove mantel.Then tile, refit mantel legs & shirt- route a notch exactly the depth of the marble installed on the inside of the mantel surround (legs & skirt) to fit slightly over the tile - install mantel, apply small trim around the edge to cover any teeny gap if necessary. Proceed with finish. Simple huh? lol

Install your mantel and legs. Then just tile up to them. It's much easier to trim the tile to fit than to trim the mantel and legs. You can always add some wood cove trim to cover where the tile meets the mantel in the event that your tiling is a bit uneven, or you just like the look. If you don't use the trim piece, fill the joints between the tile and wood with a sanded caulk that matches the grout as grout alone will crack at those joints. Remember to check the local codes in your area as to the required clearance from the firebox opening to the wood mantel and legs.