Does it matter in what order my units are in unit panel before battle? And if not, then on what priciples does computer place my units on the battlefield when battle begins?

Eh, I think it does it logically. There isn't any way to modify unit order, but what you can do is determine which units are in which stack.

Hmm...if i remember right...the units are placed in a kind ofdiamondformation and if you're attacking or being attacked from the front(at the strategic map)...
-the frist Unit of the 1. Row at the Stack is placed at middle Row, left side
-the frist Unit fo the 2. Row is placed at the middle Row, center
-the 2. Unit of the 1. Row is placed at the middle Row, right side
-the 2. Unit of the 2. Row is placed at the front, left side
-the 3. Unit of the 1. Row is placed at the front, right side
and the last 3 are placed at the rear from left to right.

It will change if you're attacked from a different direction..and then it get's complicated :)

But it isn't that important anyway because you have enough space between the armys that the enemys would need 2-3 Turn to close in if you don't move towards him, or move even away from him.
The only expection might be fast, magical enhanced superheros

Update:
I forget...if you have less then 8 Unit it changes..but it works a bit like laying down a Chain or a rope...it first places the middle Row left side, then the front, then the middle Row right, then center and then the rear..and the Unit from the Roster are taken starting at the left side

Speaking of battle formations, how does AoW:SM determine the leader of a stack when two heroes are present? You always want your warrior hero up front and spell caster hero in back, but it doesn't do it based on level alone. It makes a big deal in the shadow realm where even the lowest level unit can reach you on the first turn of combat and get multiple swings on your front line hero.

I haven't noticed any logic to it, sadly, but I'm happy to be proven wrong. It seems to like to put my most powerful warrior hero in back and my weakest spellcaster heroes in front.

I played Elven campaign last week and undead Ymbria is considered a higher ranking hero than Bormac because she is a priestess, I think is the logic, and it puts her in back.

If it works like in AoW1...the army roster is filled from left to right, 1. Row then 2. Row.
Heroes are placed first, then Rank 4, then Rank 3,etc.
if different Units of the same Rank are in the Army Spellcasters and ranged Units are placed first and for the same type..the Unit with the higher Lev.(Medal for normal Units) should be placed first

I think the idea is..to give the ranged Units/Spellcasters with ranged attacks a good chance for a free line of sight/fire
and if you're the defending party it works most the times because even flyers can't reachyou at the first Turn without magical aid (most the time not even with ;).

I installed AoW again to test a few thinks and..to be honest the Way Units are placed at the Roster doesn't work like i thought. It's a bit crazy...for the Orks..the Boltshredder(Orkballiste) is placed before the medium Cav. but for Goblins..the Wolfriders are placed before the Balliste^^

I thought i had understood the mechanic^^...but it seems i am (again) at the point it's half a riddle to me.