Traditionally the fuel is introduced by the pilot at a specified minimum RPM, but this is a function controlled by the pilot-eye-hand brain interface (only on the newer pushbutton start FADEC engines is this function fully automated) and we all know how imperfect this interface is.
With a traditional system, you need to allow some fudge factor in case the pilot adds fuel early, and also if there are system faults it gives a bit of time for the fault to be detected and displayed. So most start systems start the igniters running as soon as the starter is on (in the pre-FADEC days it wasn't too hard to ruin an engine, which is why captains traditionally handle starting).
The RPM at which fuel is to be turned on is actually bit sooner than really necessary, to let the engine help the starter bring it up to self-sustaining RPM. The minimum RPM is there because you need a minimum mass flow going to the burner can to control the flame and therefore the temperatures (these engines are "air cooled" after all and the majority of the mass flow is not actually being burned, but being used to contain the flame boundaries - some flame does make it back toward the turbine during start, as you see when the ITT spikes, but it has to be minimized, and the more mass flow the better).
You can delay introducing fuel well past the minimum RPM, and you will actually see a lower ITT peak on a non-FADEC engine by doing so (because, as I said above, more mass flow provides more flame control), but this really adds to wear and tear on the starter.
When I was in the Regional Airline business supporting CRJs we observed that a lot of captains made it a practice to delay fuel introduction quite late (like waiting to 25-30% instead of 20% for the GE CF-34) because they noticed that the ITT peak was lower when fuel was delayed. However this had to be balanced against shorter starter life and we advised against getting carried away with this because the marginally lower starting ITT peak didn't appear to actually improve engine life, while shortening starter life significantly.
On the other hand, it's important to know that it isn't essential to get the fuel on right now when the RPM is at the fuel introduction value and you can do it at a relaxed pace. If the nominal value is 20%, putting fuel on at 23 or 25% does no harm, so take it easy (pilots in training (including me) would fumble with the thrust lever lockout thinking they had to get the fuel on exactly at the right time).