IR or "Near Red" is always present, its just that the camera only starts to see it at heavier levels of ND, often from 1.2 onwards but thats not to say you won't see it at .9, .6 or even 3 under the right circumstances - just not often or its very unnoticeable. Thats why general thinking is that it's not a problem until 1.2.
The heavier the ND, the more of a colour shift you get as most gel based ND is not made from a neutral colour, but rather very heavy shades of off colour gels (purple, green, blue etc). On top of this some of the IRNDs such as Tiffen use a colour correction element (green) to combat the IR so you see that tint as well, which can be corrected in camera by introducing a magenta shift.
So if you stack a IRND that has a colour shift on top of another IRND that also has a colour shift then you will see it even more which may be visually undesirable/unacceptable. In the case of the 2nd filter being less than 1.2, you may well be able to get away with using a non-IRND which will overall have less of a colour shift so will give a better result.
Having said this, IRNDs less than 1.2 tend to be a lot more neutral than their heavier counterparts (1.2 >) anyway. IR pollution is a lot harder to correct in post than the shifts introduced by using IRND filters.
With a camera such as the F5, I would be inclined to use the internal filters to get you to a decent base level, then supplement with either ND or IRND filters on top of those. The Sony sensors seem to be quite resilient to IR pollution so you could probably get away with non-IRNDs to .9 or even 1.2
I have found that the internal NDs on the Sonys (F65/F55/F5) are all neutral and IR protected to their level of ND, but any additional external filters will introduce IR to the system - it just might not be enough to notice / be a problem.
The internal behind the lens filter module for the Alexa is also excellent (no colour shifts, consistent stop loss, full IR protection, etc).
The external filters that are the best are supposedly the TrueNDs, that use a metallic cut system but I have not yet seen them and they are very expensive to buy.
As pointed out you can also use Polas to being the level down but they do have an effect on the reflected light in the picture and that effect will vary as the camera moves relative to the light.
Ultimately it's the choice of the DOP as to what approach to take as it all effects the final look, but it pays to be informed so you can answer any questions from a technical standpoint. Also remember that if they are any good, your DIT is probably the best person to critically (technically) analyse the image as you are only seeing it through an onboard monitor or EVF, so their opinion is also valid.