2. Line of Sight
It is vitally important that you set up in a position that provides excellent line of sight for your units. It is very rarely that the Guard play on a table with a centralized and heavily fortified position in their deployment zone and no terrain anywhere else (and you had better believe that even with a Strongpoint Assault mission, things will be difficult!). Therefore, mind needs to be kept of terrain, both in and out of your deployment zone, that can block LOS.
Most obviously, you need to have clear LOS from your mass of guns to your enemy across the No-Man's Land that is outside your deployment zone. But as for terrain in your own deployment zone? This is where "traditional" tactical doctrine must be reinterpreted.
A familiar credo to players of Warhammer Fantasy is to "use terrain to anchor your flanks". In 40K, this is a mistake. Placing the flank of your army up against a piece of solid terrain (one that neatly blocks your LOS) is stupid. You might as well beg your opponent to assault you from that side, because that's what will happen. When faced with terrain in your deployment zone, you need to make it work for you instead of against you.
First, height is only advantageous if it lets your troops fire, and does not needlessly expose your more vulnerable and/or important units to retributive fire. Open your codex to pages 28-29 for an illustration of what this means. You will notice in the back of the IG lines, there is a large bunker, and on the top tower of the bunker six the senior-most officer and his Command Squad (consisting also of Company Standard, Vox-Caster, Medic, Meltagunner, etc). By virtue of being so high above the field, the Command Squad has LOS to virtually everywhere on the board. And likewise, virtually every point on the board has LOS to the fully exposed Command Squad.
Clearly, the Command Squad gains no real advantage from setting up in that position, but rather places itself needlessly in harm's way.
Second, terrain is only advantageous if it does not limit your LOS, or if it completely blocks movement as well as LOS. It is far wiser to set up entirely within wooded terrain, with your forces all within 6" of one edge and thus able to shoot out, than it is to set up outside it (with the woods blocking LOS but not movement) or worse, on both sides of it, with the woods effectively preventing each half of your army from providing supporting fire for the other (because, again, no LOS).
It is even wiser, if the terrain is insufficiently small or large to be used to a protective firebase, to set up wholly outside and a goodly distance from it. Ideally, you should set up at least 18 inches from any LOS-blocking terrain that does not totally prevent enemy movement. The 18" rule is very important, as that is how far most enemy assault units can reach in a single turn, counting both movement and assault. Other units can threaten further, and we'll cover more on that later.
Finally, remember that your vehicles block your LOS just as effectively as they block the enemy's.. Although you can move your vehicles out of the way, that means that you're reducing their firepower as a result. Save yourself a lot of aggravation, and just make sure to set up your vehicles in a place where they won't effectively deny your slow-moving infantry arcs of fire. Think of them as large pieces of terrain (which you can move) that completely block LOS -- set them up behind infantry units, not in front of them. They shouldn't be used to "anchor your flanks", but rather to protect them from a fair distance back.
As a gentle reminder, not specific just to Guard commanders, terrain can also be used to deny enemies LOS at the start of a game in your favor. A good set-up phase might help keep that enemy Vindicator (for example) from doing anything for a few turns at least, and give you the time to weaken other units with your massed guns before having to devote time and firepower to the Big Gun.