From the Fundex game manual...
WILD RAGE: When the Wild Rage is played, the player must announce
what color it will be. The selected color may be the trump color. Wild
Rage is considered to be the highest value of the color it is called out to
be. If both Wild Rages are played in a single trick and are called the same
color, the second Wild Rage is considered to be of a lower value than the first.
Translating from the German Amigo rules via Google translate...
"Joker": If you play a Joker card, you must announce a suit that is valid for the Joker. The selected color may also be the current trump color. The Joker is the highest card of the chosen color in the lurch. If the current trump suit color is not chosen for the wild card, then each card in the current trump suit is higher than the wild card. If both jokers are played in one trick, the second joker is higher than the first.
In both rule books, nothing allowing the selection of any value except the highest for a wild rage card. Your only choice is what suit / color.
Another set of rules that don’t conform to either of the manufacture rulebooks...
There are two wild Rage cards in the deck. When a player plays these, she can announce the card as any suit or number up to 16 that he wishes. By going up to 16, even though the deck is only to 15, this makes that card the strongest card in its suit if the player wishes. The second Wild Rage card cannot be played on that same trick.
In that variation you can assign the suit value above 15, which makes it a more flexible and thus powerful card. Still it says nothing about being able to assigned a negative value that would never win.
Allowing a negative value to the wild rage card with the intent that it would never win reduces the skill needed for calculating the number of trick you intent to capture. As a result, this would increase the luck factor needed to win... I could see this being a good house rule if playing with more causal players; but it doesn’t appear to be an official rule.