Do you need float?
For a lot of riders having some degree of float is beneficial. The slight loss of power transfer efficiency associated with float is, for many, far outweighed by the reduced injury risk. When a cyclist / triathlete opts for fixed cleats, with 0 degrees of float, their feet and hips are locked in position closing the kinetic chain. This increases the risk of knee injuries as the knee joint ends up taking all the pressure that is needed to stabilise the pedal stroke.
How much float do you need?
The amount of float you need is determined by a number of different factors including your biomechanics (e.g., hip, knee and foot stability), riding style and to some extent knowledge. I mention the latter as it is very important if you wish to ride with 0 degrees of float that you are confident in setting your cleats up and know your optimal position otherwise injuries are likely to ensue.