There are quite a few ways to get bonuses or penalties to a specific sense that'll refer to, for example, a 'sight-based perception check'. Do people generally interpret that as a check that includes sight amonst other senses, or just ones that are mostly or soley sight based?
The specific example I'm looking for is that I have a partially deaf character (ruled as -2 to hearing based perception checks). There are some checks that are obviously affected, like listening at doors, but quite a lot that seem a grey area, like noticing an ambush. I don't want to make the drawback too irrelevent, but I also don't want to feel like it's affecting all opposed perception checks ever.
What do people think?
It's going to vary from situation to specific situation, but as a rule of thumb, I would apply the penalty when the sense is the primary means of detecting something, i.e. if hearing it is the easiest way to sense it. When trekking through a dense jungle, it would be easier to hear a waterfall in the vicinity before seeing it, say. You could also handle these things with circumstance modifiers to perception checks, but then you're probably getting so granular that it makes more sense to use the Spot and Listen skills from 3.5th edition.
In the case of "an ambush" without further qualifiers, I would probably not apply the penalty, as catching sight of a bandit darting behind a tree could be just as decisive as hearing one sneeze.
I'd apply the penalty to any check that involved hearing among other senses. For one, a -2 penalty on a skill check just isn't that bad. For two, it's consistent. There's no feeling it out style "how much hearing did that involve?" questions, it's just "hearing y/n".