Do you need to dye pre carding or spinning or can you dye once carded or once spun?
I think you can dye when it suits you, but the optimum techniques may differ.
Stove-top rainbow-dyeing works best on raw, unwashed fleece - the colours mix and melt better when there's some lanolin there, but it will dye anything.
A friend does a lot of microwave dyeing; we've been experimenting with dyeing tops, finished yarn and finished (knitted) items - all have dyed fine, and a completely different effect to the stove-top dyeing.
The dyers who sell their beautiful colourways on Etsy and so on paint the colour onto tops, I think - that's a whole new area I haven't even opened the book on yet.
I just bought myself an eBook on Dyeing in the Kitchen in Interweave's Countdown to Christmas Sale, and some more acid dyes in an eBay seller's closing down sale, so I think 2013 is set to be a colourful year!