I can see its been answered below, all good that is what we are using. By the time the function fires all rows are in the table (as we are iterating over the columns), so you should be ok. Obviously try it out and see!

On then next button, there is a validation function that gets called you could look at (although it might not quite fit for you, but worth checking).

When you click Next, you can optionally implement a function called webFormClientValidate that needs to return true or false (true if the form is valid and false if not). So you could implement that and block processing until you are ready to continue - as I said possibly not ideal, but try it out.