In Mad Max: Fury Road, as far as I understood, Max was considered a very valuable donor. Then why did savages tie him up to the grill of a car, where he was so vulnerable and could be easily killed?
Another possible explanation is that it might have been done to induce an adrenaline rush in Max. The War Boys would then receive some of the effects when performing the blood transfusion later in this scene.
Aside from the Warboys not always being the brightest bulbs in the box, where else could they really put him?
He has to be restrained otherwise he's likely to attack and kill all of them, they can't really tie him to the roof of or hood of the car as they're rather small or lack a solid surface to tie him to where the hood is concerned, so the only place left to put him was on an oversized front grill.
Given their entire culture is based around cars and their aesthetics ("You will ride eternal, shiny and chrome!"), one could also say they mounted him as a hood ornament, similar to the way luxury car brands place the symbol of the brand very prominently on the hood of the car.
Adding to MattD's answer, I think part of the reason Max was in front was so that he'd have a good view of the action.
Multiple times in the film, it is made clear that the War Boys like to be seen, or 'witnessed'. Specifically, whenever one is about to die for the cause, they first cry "Witness me!". Once they make sure people are watching, they do whatever cool/stupid thing they were about to do. There was also that scene where Nux was excited when Joe looked at him. From these examples, it seems clear that they'd want to give Max a clear view of the action, so putting him in the front of the car makes sense.
As for why they wouldn't put him elsewhere:
Putting him in the car or on the back would put him too close to either of the brothers; since in the past Max has proven to be good at escaping, this could prove dangerous. Also, since he's in front of both brothers at all times, they can keep an eye on him.
Putting him on the top or sides of the car would limit visibility and maneuverability. He'd just get in the way. Note how most of the fighting in the film takes place above or to the sides of a vehicle, so these are the places you want to keep as clear and open as possible.
Purely tactical. They only bring Max because it's necessary. Slit and Nux argue about it, that Nux doesn't have time for a top-off and that the blood - bag is feral, muzzled because he's dangerous.
There is only 3 spots they could put the blood-bag. Nux's interceptor is a modified 1934 Chevrolet 5 Window Coupe. A 2 seater normally. But there is only room inside for the driver. The rest of the space has been taken over for a large secondary gas tank. There would be no room to securely put Max inside. And since he's feral, it wouldn't be wise.
The roof, well, as I have argued before, Max could fit like a tree or deer strapped on top, but there isn't time for that. They are short on time. The time to securely attach Max would be the same time that Nux would have needed for a top-off.
So that leaves 2 spots. The forward lancer's perch, or the rear lancer's basket. Since Max is a feral Blood-Bag, it would also be unwise to put him behind where he can't be watched. That only leaves the forward position.
The forward Lancer's Perch was already ideal, with framing that allowed someone to be tied to it. It allows them to keep an eye on Max, and prevent him from interfering.
There is no need to look into religious reasons when tactical logic explains it. The War Boys are war mongers, blood thirsty, but not morons. They do recognize logic.
It links back to the Road Warrior. The Dogs of War had the captured scouts tied to the front of their vehicles in a similar manner.
Given that there are at least ten scenes from the first three films George Miller "recycled" for Fury Road I expect that there was no other reason than it looks cool.
"Easter Eggs"? or Lazy Writer can't Think Up Anything New so recycles old scenes?