I have played with my mast tuning substantially over the years of maintaining my 44 - starting with when I had the mast unstepped and stepped for taking it through the Oswego and Erie canals.
Tuning the B+R rig isn't difficult after restepping since you generally don't need to change the tension of the diagonals unless you're making additional tuning changes. The diagonals are pretensioned with the rig unstepped in order to set initial camber (prebend). I suggest reviewing Selden's manual. I don't have it open in front of me, but I believe the manual suggests a small amount of camber in an in-mast furling mast. Too much will, indeed, cause issues with the furler, but some (look in the manual for the number) is desirable to prevent the mast from inverting into an inverted camber situation (prebending in the opposite direction, concave toward the forestay). My mast (around 55' tall) is tuned with about 2" of camber and it furls extremely smoothly.
Marking the turnbuckles before unstepping is great advice. Be sure to mark the position of the forestay adjustment as well. The B&R rig is essentially a tripod. The position of every other turnbuckle is relative to position of the forestay adjustment. Lube the threads (I use WD-40) before tightening them. Stainless steel can be finicky about galling.
While retensioning next spring, I would suggest tightening the turnbuckles gradually, favoring tension on the outer stays until the final tension adjustment. Again, lube the turnbuckles before tightening them. Go slowly, sighting up the mast from the front and sides to look for any odd or extreme shapes developing. Putting too much tension on the inner stays too soon can invert the mast camber. Use a pair of wrenches to turn the turnbuckles. Don't stick a screwdriver or pry bar in the turnbuckle to turn or hold them.
Keep in mind that, with the B&R rig, the outer shrouds and inner shrouds work together to shape the mast. The outer shrouds pull down and behind the mast, forcing the mast into a more cambered shape. The inner shrouds pull back on the middle(ish) part of the mast, acting to straighten it. Because the shrouds work together, tightening the inner shrounds tends to tighten the outer shrouds (and vice-versa). It sounds complicated, but you'll get a good feel for it with time.
You can use your main halyard for measuring camber and rearward sweep. I've found that sighting up the mast, by eye, from various vantage points around the boat can also be helpful.
If you're already happy with your tune, just mark the turnbuckles off and you should be able to hit the tune closely enough next spring.