Remember that the dragons inthe MM are 'average' dragons, and thus I would assume their ability increases are already figured in.
Also all 'average' monsters are considered to begin with 10s in every stat.
If you want to make an exceptional dragon the best way is to start with a 32 point buy(or whatever you use in your game) and add on the ability score increases due to race, and then add on the +1/4 hd ability increaases onto that.
The dragon will be much much meaner.
And who knows, if you pump the dex a bit, he/she could use combat reflexes to some good effect also.
Example: Adult red dragon 3.0 version
Stats: Str 27, Dex 10, Con 21, Int 16, Wis 17, Cha 16
So the racial modifiers would be(rounding down odd numbers)
Str +16, Dex +0, Con +10, Int +6, Wis +6, Cha +6
So to make one with 32 point buy
Str 14, Dex 16, Con 14, Int 12, Wis 10, Cha 14
For atotal of
Str 30, Dex 16, Con 24, Int 18, Wis 16, Cha 20
Plus ability increases 22hd/4 = 5
Str 30, Dex 20, Con 24, Int 18, Wis 16, Cha 21
Buy combat reflexes and watch the big boy go to town.