I realize the significance of this is going to be somewhat debateable, and despite the fact we talk about "peptides" as a uniform group of compounds they vary tremendously from one to another, so it's always hard to generalize about things like storage, or in this case, filtration and the the significance of immune reactions.
For me, if I'm getting PIP from a peptide, it's not the discomfort that concerns me, but what else may be going on as a result from that immune response.
While everyone wants definitive answers, the only thing we (and the FDA and Pharma) know for sure is the range of possibilities of injecting some deformed protein is from nothing to catastrophic. This risk is somewhat limited in pharma produced peptides manufactured and reconstituted under controlled conditions. They can guess what "worst case" scenario might be by running a simulation of all the ways a protein could break down and reform, and how strongly of an immune reaction those could induce.
We don't have anything like those controlled conditions with UGL compounds reconstituted with random BACs, at random PHs, and random concentrations (the last one is ridiculous, every peptide should have a clear "standard" for dilution ratio).
If I'm getting a site reaction, that's a warning sign as far as I'm concerned, and for those peptides especially, it's worth 5 minutes and $1 to filter the vial to reduce my exposure to some randomly formed protein that no one has ever considered as possibly being injected into a human.