In my experience, smell will only be an issue if the she's not shaved. If she's shaved and smells bad, she's either failed to shower in a while or has some kind of STD—luckily, I haven't run into either issue. So generally speaking, I'll throw a hand down her pants and if she's relatively groomed then I assume it's okay. As for me, I am obsessed with keeping myself clean and desirable for blowjobs, so I'll wash my dick in the sink of the bathroom (water, no soap) shortly before expecting to get laid. Results have been generally positive.