I'm definately all for keeping the tongue forward and flat most of the time as it opens up a lot of space in the throat.  That can be a problem when singing an "ee" vowel though as it will distort the vowel.  When I sing that vowel I feel my tongue elevate slightly with the sides touching my top molers.  I guess this is similar to the arch position you refer to.

With the "oo" vowel I suggest letting the tip of the tongue rise slightly to maintain a pure vowel.

I wrote an article a while back about articulator positions.  If you haven't read it you can find it on the page towards the bottom:

http://www.aussievocalcoach.co.....-articles/

Ian

Could I seek your input on this: Tongue positions for vowels- seems to be at least 2 schools of thought: 1) Keep the tongue in the "ng" position for all vowels (opens the back of the throat) 2) keep the tongue reasonably flat but forward for "ah", oh" and "oo", use a half arch for "eh" and a full arch for "ee". What do ya reckon? They do seem to make different tones....

T.