When not to use 厕所 when asking for the toilet? I've noticed sometimes in polite company or in the company of strangers some people will saying they are going to wash their hands 去洗手 (qù xǐshǒu) instead of going to the toilet 去上厕所 (qù shàng cèsuǒ).

Alternatively, asking for the 洗手间 (xǐshǒujiān) instead of the 厕所 (cèsuǒ).

Is this something that should be copied? Does it matter for guys, is this more feminine to say 洗手?
Roughly speaking, 洗手间 = bathroom/restroom and 厕所 = toilet.

洗手 literally means wash hands. It's not feminine, it's just more polite as you mentioned.

You can use either in most cases. You would use 洗手间 while eating or when talking to someone you don't know very well.