Im a first semester student and have been to...6 clinicals so far. On my first clinical day, I ASKED a patient if she wanted a bed bath and she said no. I realized I shouldn't have asked, rather have said "I'm going to get you cleaned up now!" Of course, I leave the door open for refusal but when you're confident and in control they're less likely to say no to you.
You have to be assertive. Of course, they can ALWAYS refuse a bedbath and there is nothing you can do about it. You can ask them why, and I like to say how good it feels to be clean. But, really, it's their choice. My clinical instructor was a little mad I didn't give a bath on the first day but she understood. Even another classmate came in to try and help me convince my pt (and this classmate ironically works in the same hospital and is a pct-she would be the best help) but nothing worked.
I suggest not asking the patient and being in control, saying this is what we're going to do. But, again, if they refuse don't force them. I'm not sure how fair it is to require you to have your bed bath done so fast. We get on the floor at 730, then breakfast is there, then VS, Meds, and then Assessment/with bed bath. I figure as long as it gets done by the end of your shift, it should be okay.
Try not to let a patient say "later" because, as I'm sure you know, things come up.