Simple answer...
1. Depends on which tradition you are from
2. Depends on what ceremony or ritual you are performing
Complex answer
1. Traditional Wicca, would say the athame should be made
of metal, iron, with the handle wrapped in black leather,
or if not that, painted black, or black cloth...
Some would even go so far as to tell you to learn how to forge
your own athame, so it would contain your energy signature.
This is as well the case in certain Witchcraft traditions, other
than Wicca, and certain Ceremonial Magic(k) traditions.
There are traditions, such as my Faery Tradition, where the
athame is made of antler handle with a copper blade.
Silver, in some traditions, has been used.
I have made one with a deer bone handle, and wood blade,
for work with certain entities.
2. What "perks" or reasons for the use of certain materials, or not?
Speaking from my Faery Tradition, my friends and co - workers, those
of the Otherworld, find iron dis-harmonic with their vibration.
A copper athame runs the Faery Current of blue fire better than any
other material and the antler handle comes from our ancient code.
(Blue Fire precedes matter and is the combined force of the astral
currents of earth, air, fire, and water.)
Iron is/was used in many traditions because it is/was believed to keep
demons or dis-harmonic entities at bay, or to open and close a portal
to the Otherside, or seal a circle.
Silver can be used to open/close the elemental doors in my Faery Tradition.
Bone contains collagen and is crystalline in it's composition. It is also
toroidal in it's structural shape and based on the phi ratio in it's natural
form. As some bone is very hollow, it can run energy without "interference".
As well, it can be filled with sand and crushed stone, or crystal if desired.
Wood is the same as silver, to open/close elemental doors.
Yet for my use, as an athame, I can also use it for psi vamps.
Now, regarding the altar placement.
Permanent altars in a room, house, temple, or outdoors can be
anywhere applicable to your tradition and/or desired by you.
Temporary altars, for specific purposes, can and are set up in specific
ways and perhaps even directional orientation regarding elemental
doors or watchtowers or geo-mantic lines of power.
If one needs to circumnavigate the altar, you are correct, it can neither
be up against a wall nor would one necessarily have room to perform
a "Great Rite" upon the altar. (enough said about *that*)
I hope that helps!