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I would actually disagree (respectfully) with Niles, in a subtle way.

First, develop on whatever version of Sage you have. Don't waste time downloading or building if you have an idea!
But once you are at the point to think about committing it, then it is worth going to the sage-release Google group and finding the latest alpha or release candidate and making sure that your patch at least applies to that. It is not correct to say that each patch builds on the released version; each alpha is based on the released version, but there are still patches that need to be rebased on other patches. There is certainly no 'back-basing' as in the update to the question.

This is annoying, I agree, but is worth doing. Many (most?) patches won't conflict, but it's worth knowing ahead of time; at least for me, rebasing is a long and tedious business.
Once one gets developing a fair amount, one will know where to look for "usual suspect" tickets or authors. Doing occasional searches in (say) symbolics will give you a sense of whether any symbolics patches are about to be merged, so that if you are going to do something in symbolics, you can be ready to base off of patch X with your patch Y.