Is it better to...

...figure out a composition as one goes, or have it fully imagined in one's mind at the commencement of the project? I remember reading a discussion on this topic a while ago, but I cannot recall the result. All I remember is that one was good and one was bad. :P

On a very similar note, supposing one was composing for orchestra and the strings had the melody throughout ('tis an example only), is it better to figure out the entire orchestra's parts at the same time, or write out the strings first. That is, should one have two bars of full orchestra or two hundred bars of strings only?

It differs from person to person, project to project.

For me, through cerebral contextualization I have the piece almost completely realized internally, before notating it.

But, I have written through composed pieces...whatever works best for you, or your particular project.

I'd think it would be easier to work with a string orchestra reduction or a reduction of some sort, and then make orchestration decisions as part of your final phase. Of course you can notate on your reduction of what you want to do to the orchestration as you go, but with the reduction you can look and see where your climaxes are and make the best decisions on how to orchestrate them to bring about the full effect that you want...

I can't work in reduction. I have to score everything fully, and then reduce it in the case of vocal works - piano reductions, etc.