There are no clear rules about this. Speed and team speed is an easy and 'safe' way to make a good rune combination for your team. Difficult to go wrong there. Leads of smaller teams often emphasise using speed, because at their level there are still too many players who just place runes without really knowing what they are doing.
More advanced players will often use more strength and also life runes. Depending on what they expect to face in battle.
Personally I often prefer to go second on attack, at least in wars, so I think of monster combinations, runes and relics that facilitate countering the most likely opening move from the opponent (which unfortunately was made quite a bit simpler thanks to SP's new ai). Sometimes this can lead to rune set ups that do not look logical at first. My Darkzgul at the moment still has team life, because I used him as a team life slave when attacking Voltaik bases with Faraday. Normally he would carry at least one strength and 2 speed or team speed runes.
Former co-lead of Resurrection of AMAB and Untamable; Retired: Parked in the Fifth Element; Make War, not Races