Are you asking if it is 100% safe? Then no, probably not. Are you asking if there are hundreds of people out there who have tried it with no problems? Then yes, there are - I have read their comments about it. As to whether you should try it, only you can answer that but let me tell you if I had a V2.0 PSP I would have 1.5 on it and be playing homebrew by now
If you do try it post your results here. I am going to downgrade my friends tomorrow on the train to work and will let you know how it goes.