I've used cold tap water forever, but with this P51D project, the decals were not coming off the sheet even after soaking for up to a couple of minutes. With the decals being as old as they are, I guess it just needed more time to loosen up.
I think warm water also is best now. I've also watched youtube videos on applying decals, and it seems to me majority rules as far as warm water. I start out with warm tap water but over time is gets cold, and sometimes I am too much into decaling to get up for warm, fresh water.
So just like you guys I also thought about using a coffee warmer, but I thought it would keep the water too warm to hot, but now that a number of you have said it does the job, I will get me a warmer.
I live in the country, so my water comes from a well, and it has more contaminates than treated city water, so I will also use distilled water.
Thanks guys for all the input.