He is WRONG.  Preheat is even more critical prior to tack welding due to the even quicker heat up of a cool part and much quicker cool down of that cool part.  The micro structure effects will be more extreme than when actually running a weld pass that has an opportunity to heat up the area itself even though the first part has not had that advantage. 

Pre-heat does so much to assist in sound weld deposits.  It is easy to find this in any book on basic welding.  The AWS Handbooks on Welding series, I think the 1st on is the Science and Technology one with much good info.  Then, in Linnert's 'Welding Metallurgy' you can find a lot of information. 

Yes, the thicker the material the more important pre-heat is and the higher it needs to be, both for tacking and for welding.  One of the main purposes is to assist with how that material cools coming down and going back up through the phase temperature area.  All of it has to do with how the HAZ will be effected as well as hydrogen being drawn into the weld pool causing Hydrogen, underbead, cracking during cooling. 

When and if pre-heat is needed will depend upon ambient air temperature (humidity can also come into play but isn't included in the codes for the most part) as that will determine if your parts are already at the required heat for the thickness encountered.  But, there are times it is advantageous to add preheat even when the parts are at the code required heat.  You can't go wrong adding a little heat but you can really go wrong by not using it when needed. 

Hope this helps.  In a bit of a hurry so may not have really explained it well.  Mainly trying to clear up any confusion.

He Is In Control, Have a Great Day,  Brent

Frenchman from Germany...
Lets leave that there,
for CS preheat is ussually recommended to limit the risk of hydrogen or cold cracking and is based on:
Heat input
Carbon Equivalent
Hydrogen content of consumable
Thickness/Restraint.

For higher alloy martensitic steels it is in general mandatory.

SO in short, if you have qualified your procedure and proven that you can weld without preheat then it should be fine, just keep in mind though it is better to stipulate a minimum temperature as opposed to no preheat or RT as this will vary between teh seasons.

Then as stated by the Gentlemen above, for ferritic material you could rarely go wrong with applying preheat for a varietey of reasons.

The amount of preheat required is a function of the moisture contained in the pores of the steel. We have to preheat submarines to a higher temperature and for a longer period of time when performing repairs because the steel hull absorbs more moisture while submerged at great depth.

And I have some dry land in the Everglades that I would like to sell, cheap.

Al