Coloured grounds, or plain white

I've been painting in oil lately, on small (7" x 5") canvases. I'd bought a pack of them, and found them useful to run a few experiments. With some, I laid down my usual burnt sienna imprimatura - coloured ground, basically. Just thinly painted on, but it's said to help with the tones; and as it happens it's a similar colour to my palette. With others, I used a raw sienna tone. And some I just left white, and painted straight onto them. It might just be a question of different subjects producing different results, and half a dozen canvases isn't a great sample in statistical terms, BUT - I have a feeling those which I left white have actually turned out a lot cleaner and fresher than those which I'd tinted. Maybe they're brasher rather than fresher, it's hard to judge as yet (takes me at least a month to work out if a painting has come off or not), and maybe the small size has a lot to do with it. But it did make me wonder what others think, whatever medium they work in; I know you'd tend to use tinted paper for pastel, and watercolourists generally use a very weak raw sienna wash at most ... how about acrylics, gouache, oil? Do you automatically tint your canvas, board or paper, or prefer to work on pure, glaring white?