If I save it as a JPG with the quality set at 100%, is that the best way to do it?
No...Yes, well... It depends.
(An edition added to the end)
Let's get a bit technical, but not much.
IrfanView uses a specific subtype of JPG compression called 4:2:2 that has lower quality over PhotoShop or PhotoPaint one that has a 4:4:4 one.
So if you started with a really high-quality JPG image you will lose information even at 100% quality settings.
The analysis is here.
Take an uncompressed image and put it over the already compressed one with a blend mode difference.
Then use levels to increase the contrast on the image.
Here is a comparison between Irfanview at max quality vs a generic one (Gimp, PhotoPaint, Photoshop gives similar results)
If your image had already some 4:2:2 compression, some compression blocks are already formed, so setting it to 100% will probably re-use those blocks.
A problem here is that a crop on the top or left side of the image can force the JPG algorithm to prepare some new compression blocks over the previously existing ones.
If you do not mind, and you can not see the diference, its ok.
If your target is to have a better ratio of compression, you can compress it more. Try 90%. I would not consider an IrfanView edit a "professional Edit, just a casual one so it does not matter.
One more thing... If it is for print... why do you care about a heavier file?
You need to try to maintain overall quality. So if it is for digital print, use PNG. If the file was CMYK use TIF.
IrfanView has a good quality compression. If you do not mind, the little loss of information, it is ok. Especially for batch crops.
Edited sometime later:
The dialog box of Irfanview has a small checkbox to disable the chroma subsampling, using then a better quality algorithm. Similar to the ones I mentioned for Ps and PhotoPaint.