Whenever a CTA clearance was simply for convenience (i.e. most direct route, trying to get a bit higher into C steps) I've found that more often than not ATC don't really want to deal with it.
The only time I have absolutely required a CTA clearance to reach the destination safely due weather (short of turning around and going back home) I found ATC to be incredibly willing and able to help, once I communicated the situation.
"Cleared direct to the field not above 4500, track as required"