Maybe. It is hard to say. There is an argument that it is better to run at a higher frequency so your N900 can finish what it is doing and go back to idle mode more quickly (where the CPU uses virtually zero power), but I think most people fail to take into account that the voltage is typically increased when the frequency is increased which affects the power calculations, this argument may still be valid even considering that, but I'm not sure.

For me, I use my N900 a lot for listening to downloaded radio shows via the Media Player, which means the CPU is constantly active at a low level, so I think it is better for my use case, since I don't think there is much opportunity for it to idle while doing that, but I also put it on charge when I can, which makes it hard to evaluate the effect on battery life.

Maybe you should try it yourself and see, one effect you may notice from switching the scaling governor to conservative is that it may be a little less responsive at times because it takes longer to scale the frequency up when needed.