Very high speed diff pairs need to be tuned for matched length all the way along, not just once somewhere in the middle. I'd like to see step-by-step length matching for diff pairs as a tuning option. You'd set the max length you were prepared to go before doing something, and specify whether to use a trombone, wiggles or both.
Why? because on very fast signals (think PCI-E Gen2, XFI or SFP+) the eye closes up very quickly if the diff pairs go around corners and the fast edges get out of step - even if they are brought back into step before the end of the trace pair.