7. Vehicles and Cover
Now we come to vehicles. Keep in mind that the Official GW Q&A expressly states that nothing blocks LOS to vehicles except terrain and other vehicles -- thus, they cannot be screened by infantry units (not even infantry in close-combat). With that assumption, any vehicles you DO use need to be carefully screened by things that CAN hide them. This typically comes down to several different things: partial cover (granting units Hull Down benefits), total cover (no LOS by enemy guns at all), and alternate threats (the enemy would rather shoot at something else).
The last item is something that falls more under the purview of tactics, and will be covered at that point. The first two (Hull-Down and Total Cover) can be part of your army's set-up, and indeed should be. Tanks are incredibly vulnerable to enemy fire, so they should ideally be put in some solid Hull-Down position and stay there.
"Hull Down" is set-up advice that applies only to durable tanks that intend to stick around and shoot from the very first turn. For other more mobile units in your army, such as Transports, Hellhounds, Sentinels, and especially Basilisks, you optimally want total cover to start. A high hill or other large piece of terrain is the best choice, but simply behind another vehicle is sufficient in a crunch (a good option for Walkers, who can move and fire their weapon, and are generally small enough to benefit from total cover when hiding behind another tank). Remember that there is never a guarantee your army will get the first turn (Lord Macharius is no longer a viable Special Character!), and vehicles are always a primary target, particularly when they are in an army as slow-moving as the Imperial Guard. Transports in general have no business being available targets from the very first turn. Don't be afraid to start them well back and in cover. If your objective isn't within a single turn's movement (12") of your starting position, the transport vehicle can fire off its smoke launchers for that one turn in which it is vulnerable to enemy fire.
Note: While your big shooting tanks (in other words, your Heavy Support Choices) are best used from the back lines of your main infantry position, in a hull down position where they have good LOS and the ability to support your troops (and vice versa), your transports (and walkers!) can set up in more remote positions, particularly if you find that they would block the LOS of your other units, or get "trapped" behind your infantry, if you set them up with the bulk of your troops. Most players will not let you rumble over your own infantry with impunity (and even when they do, they typically argue that your units will have had to "move", meaning fewer shots from the troops), so a flanking position may be useful.