Not necessarily as cut & dried as stated above. Each manager has different opinions on what works & what doesn't.
Personally, I don't value overly high tactics in midfielders, so concentrate mainly on passing. Speed is also useful in a midfielder, means he runs with the ball more & therefore won't use long passes as much. Might be better to have a mix of midfielders with high speed & high power - or just get players that are high in both!
Also, I mainly have high tactics in my Central Backs only, to counter Strikers with high tactics. Wing backs may be better having higher passing - seems my speedy wing backs run down the field too much & stuff up the final pass to the forwards - so I may need to put the passing up a bit on those guys (or get slower guys that don't run too much).
Also, once you get your training center high enough you can train some of the fixed skills to fix players with a deficiency.
But as Anomander says, check those help files & play around with your team & see what works best for you.