Remember that current color negative films have huge overexposure latitude and straight linear curves after the toe region. And, remember that the box speed does not include much underexposure latitude.
What a color temperature filter such as 80A or 80B does is, it just removes two stops of red, yellow and some green from the picture, letting all blue go through.
Now, what happens with 80A filter, is that you either measure through the filter and end up increasing exposure, or you measure without a filter and apply a filter factor of 2 stops. So, you "overexpose" two stops to get blue in right place on the curve (away from toe), and the filter removes excess red, yellow and some green so that they are in balance.
Now, if you didn't use any filter, and shot a ISO 400 film at ISO 400, you end up having blue record at "ISO 1600" and red record at ISO 400. It is hard to color correct.
Let's get back to the overexposure latitude; if you just applied a filter factor of two stops, WITHOUT using a 80A filter, you would have blue record at "400 ISO" and red record at "ISO 100". And as we know, color neg film overexposed two stops still gives perfectly linear response.
This leads us to a good rule of thumb:
If in wrong color temperature, OVEREXPOSE by the filter factor for the filter for that particular conversion, even without the filter. This allows you to do proper color balancing in post processing. The film is linear so you can do it in analog printing by just dialing in more yellow and magenta.
Without overexposure, the blue record is on the toe of the film, so you lose the linearity, making the color correction in post very difficult.
If you happen to own the filter, nothing wrong in using it. With slide film, it's compulsory if you want anything but orange.
Now, with today's very good films that have some underexposure latitude, too, you may be okay by overexposing only one stop...