There are 2 ways to go about ordering a custom frameset. One is to specify every detail, including frame angles, tubing lengths down to the millimeter, even tubing types and gauges, and then the builder, essentially acting as a technician, makes it to your specs. If you have owned many bikes, have a firm understanding of bicycle geometry, and know exactly what you want, this is a fine approach.
The other way is to choose an experienced framebuilder with whom you have a good rapport, tell him your body dimensions and biomechanical/medical issues, what you want, how you want your new bike to be similar or different from bikes you have ridden in the past, what kind of riding or racing you plan on doing with it, and leave it up to him to create your dream bike. This is how Richard Sachs works. He has definate opinions on bike fit, geometry, and aesthetics, and will not compromise his values to meet the oddball request of a customer. Read this fascinating interview of Richard Sachs by Grant Peterson, wherein Sachs explains why he doesn't "really make custom frames":
http://www.richardsachs.com/articles/rsachsriv.html . I am a fan of this second approach to ordering a custom frameset, where you choose the framebuilder, and then trust him to build you a fantastic bike.
Getting back to your question about head tube angle, in my opinion it is an almost meaningless number when considered in isolation. It is a gross simplification to say that bikes with "steep" head tube angles are "fast," "agile," or "twitchy," while those with angles like 72 degrees are "unresponsive" or "sluggish." How a bike handles in response to steering inputs is a function of many factors. An important variable is fork trail, which can be calculated from the outside tire diameter, fork offset (rake), and head tube angle. Kogswell Cycles has a handy online trail calculator:
http://www.kogswell.com/geo.php . But trail is not the end of the story. Fore-aft weight distribution, wheelbase, chainstay length, BB height, frame and fork tubing characteristics... they will all affect how a bike "handles." And the handling characteristics that one person likes might make the bike unrideable to another person. So, the answer to the question of "what is the best head tube angle for a cross bike?" is, "it depends..."
I don't mean for this reply to be condescending. If it comes across that way, sorry. It's just that I frequently see posts to internet forums like MTBR that read, "Hey guys, I am ordering a $2000 custom Ti 29er frameset, here are the geometry numbers I am thinking of... Do you think I should raise the BB by 1 cm, or maybe shorten the chainstays to 16" because I heard that's what Jeff Jones rides... And what about the top tube--should I keep it 24" or add another half inch?" I think that most people would get better results trusting an experienced framebuilder to make these decisions, rather than soliciting opinions on internet forums from people who may or may not know what they are talking about, and who almost certainly know nothing about your individual body dimensions, riding and racing history, fitness, flexibility, strengths and weaknesses on the bike, preferred components for things like saddles, bars, and shift/brake levers, medical problems like knee or back pain, pedalling style, etc., etc.