I am installing marble tiles 12x12 on my shower walls. Should I space them evenly and grout or place them together and not grout or place them together and grout the hairlines?

You need to allow some space between the tiles.
Most likely they are not perfectly square. If you butt them together you may find that they may not end up even. You should however use the smallest spacers that you can purchase as you do not want a lot of space between them.

Thank You!

Use spacers to ensure proper tile separation. Once all your marble flooring is in place, allow it to dry overnight. Use concrete (rather than the grout used in ceramic tile installation) to fill in the gaps between tiles