I have found that deleting .log and .temp files as also .old files is quite safe from my personal experience. It is true no doubt that this deletion causes loss of certain information which in any case one almost never uses, but that is it. Windows then recreates these files again if and when required. If they are in use at the time of deletion they simply don't get deleted and then one may retry deletion a little while later.
I have deleted these files dozens of time in the past without any ill effect. If you are very finicky about deletion then another method would be to delete these files and let them lie in the Recycle Bin and continue using the PC. If all proceeds smoothly for 10 or 15 days then they can be emptied from the Recycle Bin.
Having started with the Hard Disks when their size used to be around 250 megs I think I am still paranoid about saving Disk Space. In reality with today's hard disk sizes one may never delete any files at all.