When are winds given with respect to true vs. magnetic north?
The general rule is:

If you read it, it's true. If you hear it, it's magnetic.

All charts and textual sources (METAR, TAF, winds aloft, surface analysis charts, etc) use true north as the reference.

ATIS/AWOS/ASOS broadcasts, or any information a controller gives you over the radio, is magnetic.


  Wind direction broadcast over FAA radios is in reference to magnetic north.


AIM Section 7-1-11 (page 7-1-26 in the 5/26/16 edition)

One exception to the "if you hear it" rule is that a FSS briefer will read you the winds referenced to true north, since they're just reading you the charts/textual information.

(This is at least true in the United States... other countries may vary in some instances)