I have also found I have to meter differently for different colour films. I always incident meter and with, say Portra400, meter at 200. With Ektar (my favourite travel film), meter at box. Same with Gold 200.
I think it's only Portra I give an extra stop to as it just responds really well to lots of light. Always over though, never under.
As Ian says, though. Slide needs more accuracy and I tend to either incident at box speed and shoot scenes with less contrast or meter for the highs if there's too much DR.