Agree with jasonpgc, you don't really need to do exposure compensation. However, you must take note that the exposure latittude for slides is really really small, especially for Velvia and Provia. One stop either way, and you will get burnt spots or underexposed areas with no details at all. The contrast is very high. So your friend may have told you to exposed +0.5EV to bring out the shadow details.
However if you are shooting general landscape, and there's no shadow detail to speak off, you should shoot at the correct exposure. or maybe even -1/3 EV to bring out the colour.
Velvia has very saturated colour already, if you put a polariser on it, well, the colours may become too funky. Provia has more accurate colours, the polariser will help to bring out the blues and greens better.