Less than 10% of my Hilton stays are ones where I go to a property I have been to before, so I probably shouldn't be responding. But I don't think being a frequent or infrequent guest would make much of a difference to my strategy

I normally get a feel of what the typical rates are long before I confirm any plans to stay at a property, then if the prepaid rate is good and represents a significant discount to the flex rate I lock it in. If it's only a few $/£ more I'll book the flex rate and keep checking to see if it drops, until 1-2 weeks before. Sometimes, the prepaid rate stops being offered then I regret not locking it in

If one is a very frequent guest I think you would become very familiar with the typical rates and know exactly when you should confirm a prepaid rate and when you should leave it open to dropping; obviously, dependent on how flexible your own plans are.