In my experience, it would be better to be attuned by a Reiki Master. I tried to attune myself by the Grassroots website and nothing happened. But I sure felt it both times when I was attuned by a Reiki Master. lol I have also had to attune other people because they tried Grassroots like I had with no luck and when I attuned them, they felt it and are off and running.
Plus I am there for my students in case they have any questions that need to be answered, as well as to teach them what I had learned. I know that when I learned Karuna Reiki, I bugged my poor teacher to death as it is totally different in how it works compared to Usui Reiki. In fact we have become good friends because of it and now we run a reiki circle for free healing sessions here in town. :)
I am a Certified Usui Shiki Ryoho Master and Registered Karuna Master through William Rand's Reiki Organization.
Not only do I feel that I was better off paying for my classes because I was attuned and taught, versus having something and not knowing what to do with it as some places I have found on the net have done. But also I have my certificates so I am acknowledged as far as my business is concerned. Anyone who wants a class or a session can see my certificates posted on my wall, I also certify my students as well.
But everyone has free will and choice to chose what is best for them. Because I wanted to do this as a business for the rest of my life, I opted to get certified. Plus now that I am sucessfully attuned, I can help others, where when I tried to attune myself, I couldn't do anything because it didn't work for me.