Is it safe to say that all Attributes....

...are best at level 12. I ask this because on Guild Wiki [not official]] all skills show that when their governing attribute is at 12 it is bold faced font, the 12 that is.

So is it safe to say all attributes should be at 12 if anything??

Well, you don't have enough att points to max out all stats. But yes, skills are more effective the higher their governing attribute is...

There is no possible chance you can get all your attribute to 12. You can get 2 of them at the most. If you have a 9+ in the attribute then your are good to go. You can put runes in for higher points, especially if you are a 55 monk. Other than that just stick with putting them where they are needed. Look at what skills you are using and change your attribute points around a bit to make sure that your skills still work well, and that you meet the req for your wep.

Not true at all.

12 is the value you reach without modification (armor/runes/gear/skills).

For some skills you will want to keep them past certain breakpoints (13 - 16).

Some skills can be further buffed to 17-20.

Some builds might benefit from a certain attribute being low.

There is no rule at what an attribute should be at. It always comes down to the build.

lets sum it up this way:
12 is max without mods
16 is max with mods(+1 from headpiece and +3 from superior rune)
18 is max for specific attributes, Blood Magic and Curses with Awaken the Blood(Blood spell) and all Elemental Magic attributes with Glyph of Elemental Power
17/19 is max with a +1 attribute(20% chance) weapon mod
that's it I think, correct me if I missed something

In some ways you need to consider the skills you use before you consider the attribute allocation you need to give.

Some skills have a linear progression, so every extra point in the relevant attribute will garner an increase.

Some skills have a stepped progression, so for example, 3 in Smiting may be one step for Balthazars Spirit, then 8 in Smiting may be the next step, and 13 in Smiting may be the last. In those cases, there would be no point having 12 points total in Smiting if the only linked spell you had was Balthazars Spirit.

Once you have an understanding about the difference the attribute allocation makes, then you can consider whether 12 is necessarily most efficient.

Bear in mind also, that two common attribute allocations are 12-12-3 and 11-10-10 (1). Now the two 12's are clearly good, but 11-10-10 may be more realistic if you are using skills from 3 different attributes.

I recommend looking at your skill selection first and then considering the attribute allocation.

I believe 21 is maximum possible for Curses, Blood and elemental magicks.

12 base + 1 headgear + 3 superior rune + 2 Glyph/AtB + 1 from offhand + 1 Blessing from shrines + 1 Canthan New Year's 'Lucky' blessing

Yeah.

:EDIT: Camby, (good) Warriors often run 12/10/8 or 12/9/9, maxing weapon mastery. An important one to know...

Any attribute you pump past 12 is more powerful yes, but it's the law of diminishing returns after 12. Also, some skills don't change values on every point.

Fragility spiking mesmers with Virulence is an example of a low att build.