My copy of the PRMs says it's highly problematic to multi-task during printing.
Multitasking is incompatible with the queueing mechanism used by the Printers application. If you call Wimp_Poll whilst printing, the printers application will assume that you have finished; if there is another job in the queue it will continue with that, which will obviously cause an immediate error. Future versions of RISC OS may address this problem.
It is difficult to arrange your printing such that you call Wimp_Poll sufficiently often to confer a useful degree of multitasking on the rest of the system, since you cannot predict for all classes of printer the size and complexity of rectangles that you will be asked to print.
To do so properly, you need to use a timer.
You must select the previous print job before polling the Wimp, and reselect your own job on return.
In practice these problems are likely to outweigh any advantages conferred by multitasking. If you have any doubts as to whether or not you should multitask, WE RECOMMEND THAT YOU DON'T [my caps]
So where in the PRMs did you read the recommendation to poll?