Flags for trig representation (positive frequency*t+phase)?

[parisse]

There is no way for the CAS to guess that cos(2x-3/4*pi) is better than cos(3/4*pi-2x). If you want to negate all cos arguments in an expression, you can run subst(expression,cos,x->cos(-x)).
cos(-x+pi/2) is auto-simplified to sin(x), then trigcos will not do anything, because trigcos applies the relation sin(x)^2+cos(x)^2=1 and nothing else (if you run trigcos(cos(-x+pi/2)^2), you will get 1-cos(x)^2).