pre-soaking tea in cold water prior to brewing

I don't think the couple minutes of soaking is actually doing anything; it'll pull a bit of stuff out of the leaves, and get them wet, but what really matters is the hot water. It sounds like this is a way of getting lower temperature water, similar to your proposed "protect the tea from hot water" explanation. This is indeed good for green and white tea, and maybe oolong, but essentially unnecessary for most other teas.

You don't actually always want boiling water for tea. Joe provided this table of temperatures in his comment. Some temperatures for common types of tea, in decreasing order of temperature: maté, rooibos or herbal (208F / 98C); black (195-205F / 91-96C); oolong (195F / 91C); blooming (180F / 82C); white or green (175F / 80C). So for some teas (black, maté, rooibos, herbal), it's pretty close to boiling - by the time the water's poured in, and transfers some heat to the cup, it'll be a few degrees below boiling, so you don't need to worry about it much.

But other kinds of tea (green or white tea), you ideally want to add somewhat lower temperature water. If you have a way to get water somewhere around 80C - for example, some electric kettles can automatically turn off at a lower temperature - then just do that. But if it's easiest to make boiling water, then if you fill your cup a bit less than 1/4 of the way with water at room temperature (20C) then fill it the rest of the way with boiling water, the result will be around 80C, just right for green tea!

Brewing tea is pretty simple, just steep the tea in hot water for a few minutes. But you can still learn some important basic information beforehand. What temperature to brew at and whether you should choose loose tea or tea in bags are two very important things to know about. Here is the link http://coffeetea.about.com/od/teabrewing/a/teatemp.htm As it varies with the type of tea.

I suggest you to use loose tea instead tea bags, i have experienced both and realized that loose tea has delightful flavor and fragrance of tea. There are some excellent bagged teas out there, made up of whole tea leaves.Here i refer you a link what is the reason behind it. http://coffeetea.about.com/od/teabrewing/a/looseorbag.htm

Here's how use to brew Time Required: 15 minutes Recipe:

Heat water to just boiling. You can use a pot on the stove or a tea kettle. Place tea bag in your cup, or a tablespoon of loose tea. Pour hot water over tea, to fill your cup. Let steep for 3-6 minutes, depending on the kind of tea and your taste preferences. If you used a tea bag, remove it. If you used loose tea, you can strain out the leaves, or just let them settle to the bottom of your cup. Add sugar and/or milk, if you like. Drink and enjoy.

Tips:

Green teas should steep for 2-3 minutes, and black teas for 4-5.

I got enough better results by my recipe.but surely you can try it out i also tried your brewing trick and didn't found much differences. So, Pre-soaking of tea bags in cold water is not prior to brew.