How to calculate the orientation of planets at current epoch
The IAU WGCCRE (Working Group on Cartographic Coordinates and Rotational Elements) seem to be the authorities on this.
Table 1 in their 2015 report gives the following information for the major planets:

α0, δ0 are the ICRF right ascension and declination of the planet's north pole. If the planet's axial precession is known, these are expressed in terms of T, the number of Julian centuries since epoch J2000.0.

W is the rotation angle eastward from reference point Q to the planet's prime meridian at time d, where Q is the intersection of the planet's equator and the ICRF equator at RA α0 + 90°, Dec 0°, and d is the number of days since epoch J2000.0.


They define a planet's north pole as the one pointing north of the solar system's invariable plane.
This means that W decreases with time for Venus and Uranus.
Their figure 1 appears to indicate Q's RA as 90° - α0, in conflict with the text. I believe the text is correct.
When you are ready to validate your calculations, the S&amp;T Mars Profiler shows which side of Mars faces Earth at a given time.