Stipend isn't the word you want. Anyhow you don't have to do anything to "reserve" the use of 170.6. You either use it in the time frame permitted. There's nothing you need do to make that declaration within the allowed time and there's nothing you can do that allows you to hold off making it beyond the specified time. Either you have cause to ask the judge to recuse or you don't. You don't get to hold it in your pocket and spring it later if you don't like what's going on.