Quality vs Quantity,Some free worksheets are not good quality - the pictures are fuzzy, backgrounds print grey or speckled - and children tend to notice these things. If you are using the worksheets to educate your child, you may want to choose good quality worksheets that encourage your child to produce good quality work. After all, its a little difficult to ask your child to color within the lines and work neatly when the worksheet they are filling in hasnt done the same.
A lot of the websites charge high dollar amount for these math worksheets. Considering your willingness to spend the high dollar, you run in to a new problem - the uniqueness of the material, a lot of websites offer sheets packed with identical problems leading the child to almost rote memorization of the problems and solution. This leads to deprivation of student and his/her ability to solve the problems logically. And some websites offer sheets in portable document format. And to open such sheets you are forced to download special software.