Only reason to trim it is if they have issues with penis retracting back into the sheath after knotting (from sex or simply just full erection). Most males in my experience have issues with the guide hairs getting caught up inside the sheath upon retraction of the "deflating" and retracting knot and shaft to some degree or another but most deal with it by just continuing to tend to the situation calmly and the situation takes care of itself the way nature intended them to. No biggie. But some males have problems at this point and,
even with the application of lubricant and manual assistance, it can become problematic for them and in some cases gets bad enough to where the sheath is folding into itself. When this happens, if they are particularly hypersensitive to this discomfort and/or don't give themselves the proper post-sex self-care required, they may yelp until the erection has fully subsided and retracted into the sheath, which is something I've witnessed sometimes. It's definitely something to help them avoid.
Those dogs I had to trim because not even applying a ton of lube and manually trying to keep the guard hairs from getting caught up was keeping them from yelping in discomfort. That always solved the problem.
Hot tips:
KY jelly lube can get rather sticky when allowed to dry even slightly so if your male does not do sufficient post-sex self-care or it's not being kept wet constantly during retraction, avoid using that as it can exacerbate the problem or add risk of it happening unnecessarily.
Alternatively, a great natural lube I've discovered for dogs is natural coconut oil! There's unhealthy forms that are refined and only meant for cosmetic use, so make sure it says on the label: food grade, organic, unrefined, and raw, and the ingredients on the backside of product should say only "Ingredients: Organic coconut oil." No other additive ingredients including preservatives! You may also need to first melt it with your hands because the natural stuff solidifies at room temp.
This is the only truly safe oil-based lube for dogs I'm aware of, and the best lube for this application for several reasons. This natural form of coconut oil is actually very healthy for them (and humans too) internally and externally including on sensitive areas like mucus membranes. It's a natural, antifungal. They also love to comsume it, which makes this a perfect lube to apply to his cock afterwards including all around the front and inside the sheath as much as feasible, since this will encourage any self-care slacker to lick himself consistently until fully retracted like he should! This alone has helped solve this problem with some males.
So I would say, if this happens even once, avoid letting it happen again to sour their sexual experience by either trying the coconut oil method above or just skip right to trimming those hairs, carefully. Ideally with safety scissors (rounded tips) or an electric trimmer with the shortest trim guard attachment you have, otherwise you risk injury! No need to trim off completely, you can leave a few cm of length and it'll be fine.
Why some dogs are prone to this issue can be one or more of the following reasons:
1. They aren't properly licking themselves during retraction to keep the existing lubrication slick and brush away the hairs from getting caught up inside the shealth
2. There wasn't sufficient lubricant (natural or artificial) on their exposed penis in the first place during and after sexual stimulation in whatever form
3. They are particularly hypersensitive to this common genital guard hair issue that happens often to some degree or another with males after sex or after any full erection.
