Whilst lowcostdolly says that the SCCM would have been aware that wheelchair passengers would have been on the flight because it would have been communicated by the ground crew, as an ex SCCM for easyJet, I can categorically state that this is not always the case.
I always used to ask the despatcher at the start of the turnaround "Are there any wheelchair pax on this flight?" and the despatcher would either state "yes or no", if they stated "yes" I would reserve an appropriate number of seats at the front of the cabin (and I would always make a point of getting the despatcher to contact the gate to find out how many pax were in the wheelchair group or groups so that the remaining pax on the flight could have maximum amount of seats to choose from).
Quite often though, when the despatcher said "No wheelchair pax", you would have a wheelchair arrive at the last minute (Normally more often than not at an outstation and not an easyJet base). This then leads to a problem where you have to move pax on Row 1 DEF and is embarassing for the wheelchair pax and the crew and the pax on those seats who quite rightly want to sit together.
If I had any doubt in the despatcher saying "No wheelchair pax" I would reserve 1DEF at the start of the boarding and say to the speedy boarders "We may have some wheelchair passengers on this flight, but the ground staff are not yet sure" and I would invite the speedy boarders to sit on row 2 DEF or the overwing rows. If they sat at row 2 I would then bring them forward to row 1 as soon as I had confirmed the headcount. Doing this also then meant that if there were no wheelchair pax, I would have 3 seats together on row 2 which could then be occupied by the "last minute family" that always seemed to board who had been distracted either by duty free or had been delayed in security.