I have never bothered to rinse my paper coffee filters before use, but some strongly recommend this. I did some experiments, outlined below, to test this. I don't find rinsing makes any difference, but there might be benefit that I don't understand.
My questions, in sum:
- What's the best way to rinse a paper filter? Inside, outside, both? Soak? Hot or cold water?
- How does it help?
- Bonus: What is being rinsed off? Are loose bits of paper really coming off the filter?
Of the many articles about this, most of them simply point to this Serious Eats article (e.g., lifehacker and the kitchn don't have much more content; others are chat/discussions without concrete conclusions or references). That article says rinsing removes "dusty stuff" from the filter (that suggests that there's off-tastes from the paper dust), and rinsing also helps reduce clogging of filter pores (presumably also from the "dusty stuff").
The quality of the paper filter certainly matters; comments and responses to this question support that. I buy the cheapest brown filters available, so I think it's unlikely that my filters "don't need rinsing" or so, but perhaps they're so cheap as to "not benefit from rinsing." This article suggests some filter taste-tests, so I did them and more.
- I poured water through rinsed and non-rinsed filters and tasted the water that went through the filter. Both taste faintly of paper, but I see no difference;
- I have brewed simultaneous batches with rinsed and non-rinsed; I taste exactly no difference;
- I have even gone so far as to chew on rinsed and non-rinsed paper filters to see if I could tell a difference. (Edit: to be clear, I chewed on these filters only to see if I taste a difference between rinsed and non-rinsed paper filters, not to somehow "condition" the filter before brewing use)
- Paper filters taste terrible -- mine anyway -- rinsed or not. I may have to revisit this question on brown versus white filters, buy better paper filters, or use a cloth-filtered Nel pot.
- I wasted a lot of filters doing these tests.
In fact, after using a pre-rinsed filter I got a "structural failure" -- the bottom of the filter broke -- when brewing pour-over with a heavily rinsed filter. I'll probably have to eat some better-quality or white paper filters to make this fair.
In sum, my only conclusion is that quickly rinsing the inside and outside of the filter is the only way for this to have any benefit. I'm discounting any other benefit of pre-rinsing, such as rinsing with hot water would warm ceramic cone filter-holder or pot/mug.