For infrared, the safest approach is to shoot RAW and set white balance in post. IR causes extreme color shifts, and auto white balance often fails because the camera mostly sees strong red tones. RAW also avoids baking an in-camera white balance into the file.

If you want a usable in-camera custom WB, set it with the IR filter on. For standard or deep B&W-style IR, many shooters use green foliage or grass; this tends to place leaves/grass near white or gray in the final image, which is often the desired IR look. A white paper target under natural light can also work as a custom WB reference.

For IR filters that pass more visible light (often sold as super color/super blue styles), a gray card is a better custom WB target.

So, in practice:

• Best flexibility: RAW, adjust later.
• Best in-camera WB for typical IR: custom WB from foliage/grass with the filter on.
• For mixed visible/IR filters: custom WB from a gray card.
• Auto WB: generally not recommended.