[vidarh]

Depends what you want: If you want to find new processes and compounds that you can patent, then avoiding older patents is essential, as they're prior art.

On the other hand, if you want to find new processes and compounds that aren't patented, then finding a past expired patent covering a part of the process would mean that part of the process is "safe" (not finding one might mean it is safe, or might mean you've overlooked something)