fwiw, from THIS agency driver’s point of view…
My agency is a small affair, with 2 regular clients and a few occasionals.
My opinion is that if I want to keep being invited back onto a job I enjoy and quite frankly is a cushy number, then I should ensure there’s no grounds given to prevent me from doing so.
On both jobs, If there’s greasy tangled soosies, I take a photo with my camera-phone (upgraded especially to a Sony W800i so the quality is high enough to print if needs be). If there’s grot in the cab, I take a photo. If there’s damage to unit or trailer I take a photo. Anything whatsoever, I take a photo. The phone’s time isignia is set by the network so I can’t fiddle photos to show a different time of being taken, so it’s as good as guaranteed.
On one site, anything significant I make sure another driver, or a shunter, or the transport office know before I go, to prevent any problems.
If the soosies are REALLY gunged up as is often the case, then I couple up, then walk back for a fresh set so I don’t slop goobs of black slop inside the door pockets or under/beside the seat, and cos it makes my point that I’m unhappy with having to drive a dirty wagon.
The other job since there’s rarely anyone about and I’m using a salaried driver’s regular wagon for the run, I keep the same standard but fortunately they’re usually in a better state in the first place - but I make sure I take photos since there’s rarely anyone to point it out to.
Of course when I return on either job, they’re fuelled up, cleaned outside and in, and left as I found them. If it as filthy inside before though I’ll leave it as such, but with nothing added. I’m not there to valet them!!!
The drivers working on the same agency and same jobs as me are largely the same, without the photos. Many are ex-squaddie and take pride in making sure their equipment is clean and our gaffer is ex squaddie so he likes to ensure his reputation is good by drivers being clean, smart and doing a clean efficient job.
As a result his drivers get good regular work most of the year, and during lean times like now, we get priority over other agencies.
It pays dividends keeping clean. Plus, seeing as I’m paid hourly there’s no reason to moan about an extra 5 or 10 minutes spent cleaning out. Another couple of quid in the bank! Worth about £300 a year!!!
And still, if there’s any gunge where it shouldn’t be, it’s us agency monkeys who get the blamed by the regular drivers 
From what I see regularly on the site that shares wagons between drivers 1st come 1st served, the regular drivers couldn’t care less about keeping them clean since they consider themselves unsackable unless they crash a wagon, or thump someone and know they’ll probably have a different ride next shift and someone else will clean it out before they get it again.
but granted, some agency drivers are awful and poorly disciplined - but then they must live a life of working dozens of different jobs, no regular faces to get to know, and getting hassle from the agency about stuff they’ve done wrong.
I’ve only ever had hassle the once and I had photos to prove the wagon was a mess when I set off.
ps - top tip for the cleaning…
stain devil for grease, always use biological washing powder/liquid, and with plasticcy materials like hi-vis storm jackets, even a home jet-washer/pressure washer can be used (from a distance mind and not while wearing it, they’re high pressure and can cut material and flesh) for old worn-in stains.
also for grease on skin, steering wheel, handles etc, get some tesco deodorising wipes intended for women what like to stay fresh smelling at work when otherwise they may niff a bit after a long day. a bit like “sure deo-wipes” but cheaper and more wipes in a pack. lifts the grease right off and not as wet and sloppy as baby wipes.