Hmmm, well I believe it would depend upon the dojo you train at. If the dojo norm is to Kiai, then by all means do so. If you are visiting a dojo and it is a rather quiet one which does not promote the liberal use of the Kiai, then do not.
We do not Kiai during my classes. When a new student came to class one day, and began to Kiai during rolls, breakfalls, and striking, the other students began to look at him rather funny. He promptly stopped being so vocal.
So, I really think it's a matter of dojo and personal preference. Personally, I prefer to only hear the sounds of people rolling, falling, and tapping the mat during a pin. Oh, and the sound of bokken and jo coming into contact with one another is pleasant also. Have a good day!