In my own experience I have scanned with almost all the programs i.e. registry clean software available in the market but most of them only just claim to be the best registry clean software or fixer. The story is just opposite and the program does not change much performance rather sometime it may create a problem to the computer system if it is not well tested or if the registry clean software removes any such entry that might be of use for windows. Thus please be aware of the fact that if buy any such program that is indeed the best registry clean software or its just an advertisement only?
A best practice we can follow while altering a registry value if it is needed by manually or with the help of any registry program, just take a backup before doing anything with it. Taking backup of registry will always be a best practice and safer in all the way. If in case any value gets changed, you may restore it just by double clicking on the back up you have created and it will be restored and overlap/revert the changes you have made or your registry clean software has made. I call it a safer way always.
Now lets come to the point where we were before i.e. discussing about the best registry clean software. As I have said most of the programs that are registry clean software or registry repair tool or registry fixer etc they are in many names but functioning is the same i.e. to alter the or repair registry and it aims at up to date registry. However most of them proved to be NULL in my experience as they do not increase the PC performance what they claim to be or to do. But there are certainly few of them that might be of great use especially you have not got any expertise in windows registry and you do not know what to change and why. I would also suggest here please never change your registry unless you are 100% sure about the issue and you know what may be the problem and also consequences if something goes wrong.
We can segregate these registry clean software on the basis of what they scan and fix. Actually when any registry clean software scans a PC internally it segregates the white-listed items that are not malicious to the PC from the one which seems to be a faulty or malicious one. On the basis of this mechanism it works and finally removes the entries that have been identified malicious or not needed by windows for its normal functionality. However there may be occasions where it is really difficult to differentiate between the programs and it seems its a malicious or not needed and consequently the registry clean software deletes it resulting in malfunctioning of windows or related program in the windows. Hence a concept of taking backup comes in and we will be in safe position irrespective or any abnormal situation.