General rule of thumb for angles in cutting tools - the sharp angles cut easy but dull angles last longer. In sharpening my chisels, I choose 28* because "sharp" would be 21* and dull would be 35*. Sharp 21* would constantly break my cutting edge while a 35* angle would dent the wood, not cut it!
As for marking tools - I like Al's advise and follow my chisel guide line. A 28* angle on one side and flat on the other will easily mark your projects and be strong to last a long time!
Its' never hot or cold in New Hampshire... its' always seasonal.