Working interviews are a good way to see if the interview candidate has the basic necessary skills for a job, especially where a resume description of the skill may not suffice. Typically, the working interview is no more than a day or two, but in the case of a groomer, I think the interview would be no more than a couple of hours.
The important thing to remember is that any time someone is put through a working interview, they are your employee during that period of time – you must pay them at least minimum wage for the hours worked (“interviewed”), or offer some form of worker’s compensation, such as a gift certificate to your business (if applicable) or another establishment. To be absolutely safe (and in line with L&I recommendations), I would go with minimum wage-payment as it is a minimal cost and an easy preventative measure in case of any potential future dispute. Working interviews are in slightly gray area of employment regulations in Washington, but they are legal (and almost 100% expected in certain industries, like in restaurant/cafe kitchen positions). In addition, (and there have been instances of this in Seattle itself) if you do not end up hiring the candidate after an unpaid working interview, the candidate may actually be able to file an unemployment claim, and you could be held liable as his/her last employer (depending on how long the interview was).