I don't know what the rules are, but as far as safety goes it depends on the set up of the bow. The 5 grns/lb rule is for an IBO set up. Which means 30 inches of draw and 70 lbs of draw weight with a 350 grn arrow. If you decrease draw weight you can decrease arrow weight accordingly. However this is still assuming you have a 30 draw length.
If you shoot a lighter set up like a bow with lower energy cams or a shorter draw length you can safely go below 5 grns/lb, or if you have a lot of stuff on your string.
Here is a link to the minimum AMO arrow weight you can use. Remember this is AMO weight which is heavier than IBO.
http://home.att.net/~sajackson/amochart.html
Lets take my darton for example. It has 50 lbs of draw weight at 26 inches of draw. I have the older CPS cams with an IBO of around 298 fps. I think for this chart that would be a speed cam. So I can safely shoot 195 grn arrows according to AMO standards. This is considerably less than the 5 grns/lb standard. Now my Bowtech would be a slightly different set up. I shoot 55 lbs with it at the same draw length, however it has a much faster cam on it. It has an IBO speed of 324. I might want to take that into consideration and err on the high side, say go to the next block in draw weight choices or something. That would put me at 259 grn of arrow weight, which is still less than the IBO standard.
Basically if you shorten your draw length or go to the lower end of your weight range you are decreasing the amount of power your bow can develope. A 60-70 lb medium cam bow set at 60 lbs with a 27 inch draw length will simply not produce any where near the energy a 70 lb bow at 30 inches with a speed cam will. So why would you need the same weight or spine arrows? You don't.
Technically you should be able to shoot what ever weight arrows will let you achieve the advertised speed of the bow and it should be able to take it, since it is producing the same amount of energy either way. The shock and vibration should be equal with either set up. I don't know if I would suggest it though, but on paper it is sound.
Personally my opinion is that the only reason you should shoot light arrows is if you are competing in 3-D and the extra speed will actually gain points. I would make sure that the reduced weight actually gave you enough speed to make your trajectory better though, you might be surprised in some cases. It takes quite a bit of speed to actually make a difference sometimes.
Other wise I would shoot heavier arrows. They seem to fly better for me, are quieter and much better on your bow. Where the above says I can shoot a 259 grn arrow, and I would probably get above 280 fps with it I choose to shoot 450 grn arrows, especially for hunting. I don't shoot 3-d though or worry about range estimation. I practice the way I hunt, and when I hunt I don't guess at yardarge so speed and arrow weight is not a concern for me.
Good luck,
Paul