If the putty is really hard, it is better to do the whole job. When it gets to a certain point, it loses its ability to flex, and you start getting pressure cracks. I look at it this way, glazing putty is an "expendable," it's not supposed to last for ever.
Mr. X patched the putty on the windows on the ex house instead of replacing it, and we had a fair amount of trouble with pressure cracks: if the glass didn't break, then the old putty kept cracking, since it was at the end of its useful life anyway.
One of John Leeke's tips at the window workshops we attended this spring is to use wiggling motions rather than scraping and hacking. Work along the plane of the glass surface, not down against it.
Last year Himself insisted on taking on this project, hated it and ended up taking the sashes to the glass place. I've got four windows to do this summer, I'll be starting after vacation (whoo-hoo! next week...only bad thing is I really want to stay home and work on my windows.
I must be nuts...)
Oh, and after Mr. Leeke's window workshops, Himself has been converted. He no longer thinks I'm totally insane for wanting to save the windows.