WotC really should have included an ECL for every creature. That way, we could use the ECL for cohort purposes.
Anyways, I look at it this way: If the cohort would be as effective or more effective than other PCs in any of the major areas (spellcasting, melee fighting, ranged fighting, dealing with traps), I don't allow it as a cohort. Cohorts are not there to steal the thunder from other PCs. If, however, they'd serve a purely supplemental role, they're fine.
If the tiger would be equally or more effective (after spell buffs, equipment, etc ..) than the PC melee fighters, tell the player that it is too strong and suggest an alternative. If the other PC melee specialists are stronger than it, then it is a fine choice. Tigers tend to be pretty strong fighters ... I place an awakened tiger with spell buffs and equipment on par with an 8th or 9th level fighter. That means that I would probably not allow it as a cohort unless the party was around 10th to 12th level.