It varies on what your side is and what they are comfortable with. I wouldn't start with Cook on 5 aggression, but I would with Strauss.
At Sussex, on my save, I have a pretty sound OD line up, it's almost perfect. I've got Gatting and a regen opening. Gatting scores quickly and has 4 tons in two seasons, the other scores slower but gets lots of 50s. Gatting is at about 90%. I then have Joyce and Goodwin coming in (Joyce recently dropped for Hopkinson), both capable of tons. I then have Morgan, Wright and Hodd who can hit runs at a good rate.
Don't be afraid to settle down for 10 overs. I often have my batsman on 3 bars for 10-15 overs if I lose early wickets, but I can still post 250.
Pitch conditions, form of the oppo bowlers affects matters as well. If I don't lose a wicket in the first 15, I expect to be at 75-0 or better. Also, note who is bowling, if they have a bowler with 3-10 from 4, don't go aggressive against him, be aggressive against his bowling partner.