I think this (individually roasting single origin beans and post-roast blending) is a great way of being able to experiment with blends. I used to do this exclusively as well.
These days, I look at the following factors:
* Do I want to alter the ratios of the constituents as I try out the blend?
* Do any of the blend constituents require special attention? (eg if you take it into 2nd crack it loses all of its personality)
* Do all the constituents roast at the same rate?
* Do I have enough time to roast perhaps 4 batches separately?
So in the end, if you know what youre aiming for you can save time, but you may end up with a compromise result.
I have a blend that I roast in two phases. Two beans go in at the start, then an additional 2 get added about 3 mins later. The first two take a little longer to roast, and I also prefer them to be a little darker roasted, so this gives me about an ideal result in a one-off roast, saving me time.
I know there are (commercial) roasters out there who use both approaches.