I know I am way to late posting this to benefit you but thought I will still pitch in for the future generation.
-Would I run into other people in the hallway or something?
The SPs will be on the look out for you so you shouldn't run into anyone. I have visited all the agencies quite a few times in the past few years and only run into someone once with a very new SP (kinda my fault as well since I was leading the way). My advice is that you always let the SP lead the way and you walk behind the SP. That way if there is someone around the corner, the SP can signal you and the other party to turn around.
-Would I be deducted the time that I 'waste' before the session? (if it is then i wont waste precious time solo showering)
The official answer is no: you won't be deducted for shower time as all agencies and independent service providers start the clock after your shower. However the unofficial rule is to limit your shower time to about 5 minutes to avoid holding up the line. (2 minutes rinse, 2 minute dry-off, and 30 second walk from/back to the room - Shower done in 5m.). That being said, I would say it takes me about 10 minutes 50% of the time just because I am not familiar with the shower (waiting for the water to warm up, couldn't get the temperature right, can't find the soap, drop the soap, etc... ). No one ever mentioned anything so I said try to stick to 5 minute but if you are a little over 5, don't panic and risk killing yourself in the shower by rushing about. Just finish up quickly and you are all good.
-If not is showering frowned upon escorts as its basically delaying the appointment for them and I'm using their amenities for free.
This is an excellent question and you have to trust me on this. Every reputable service provider will be very pleased that you take a shower right before the appointment. Even thou they might not directly express it verbally, they definitely communicate this orally and physically.(I know it sounds cheesey but it's true). My advice is that you always take a shower at the incall before the appointment starts. By doing so, you just increase the pleasure you are about to receive by at least twofold.
-How would i bring it up casually when I greet her? 'hey can I use the shower?' What happens after that?
Hand over the donation first to the service provider discreetly and then ask her if you could take a quick rinse. She will either ask you to follow her to the shower right away if she know no other appointment is taking place at the ageny or she will ask you to wait in the room first for her to clear the hallway. Then she will lead you to the shower. The SP will get you a bath towl or if your prefer a bathrobe as well, wait for you outside the shower room until you are done, and then lead you back to the room. After they lead you back to the room, that's when they usually start the timer if they use the timer system. I would imagine shower after the session is off the clock as well but I couldn't really speak for all the agenies as I am not too sure what's their policy on after-appointment shower. If I take a shower after the appointment, I usually use the paid-time to do so. That's usuaully because I have other meetings to go to directly after the visit. I have had showers after my appointment time was up and no one ever mentioned anything so I am guessing it is okay as well as long as you don't take forever in the shower. It is best you ask the agency directly before hand if you want to have a shower after the appointment time is up.
My last thought on the subject is showering with the SP. It is a fantastic experience. I probably don't do it often enough but it is one of the most wonderful thing to do if you find the right partner. Try it. Does it come off the play time? I have no idea but I don't even care if it does because it is just too amazing.