Audience & scenario
Being a CMS system, you are not targeting casual users like the chubby woman that purchases cat chow online, but people (not necessarily smarter than said woman) that use the UI as kind of a working tool, over and over, right?
Also, things (like how to select a row) have to be learned only once, right?
Thus, a user might be intrigued about how to select a row the first time, and use this knowledge many times in the future.
This is a desktop application, right?
Then the power of #hover# is available.
When your happy user is hovering a link you make the pointer look like the well known hand of the links.
Else when the user is hovering a part of the row when she can click to select it, you switch to the arrow decorated with a plus sign to communicate that selection is available.
Additionally, when the pointer is over an already selected row, you can use an arrow decorated with a minus sign to say that by clicking there the row can be deselected.
If the whole row changes its background color immediately, your user gets the feedback she expects and gets even happier.
Oftentimes the rows widths exceed the window's.
If you placed the checkmarks at the beginning of the rows you would be forcing the user to scroll horizontally, and she wouldn't be that happy any more.
Moreover, if she is selecting the rows based on the content of a column that's far from the row start, then she could make a mistake due to parallax error and now her boss will be unhappy too.
Ideally the users would look and click right there in order to be safe (and happy).
If you need multiple lines selection, the ideal is to do it by clicking a row after the other, no control key down.
The contiguous selection idiom (select one, then other holding shift) should be available.
You can add a first thin (square cells) column that, when hovered, shows a checkmark background image, and a title like "click a row to select it" to educate the fearful users.
That cell, when hovered, would also change its background to mimic that of the selected row (but only this cell).
This way you keep both dinosaur and millennial users happy (I'm a dino).
Also, clicking the header of the checkmark column can be used, if useful, for selecting or deselecting all.
By all means, do not force your users to click in a tiny 12x12px box in order to select a row!
Yes, I'm talking about the checkbox.
And don't add offense to insult by forcing them to scroll horizontally!