Is unnecessarily staying in a more expensive hotel and charging it to my employer a sin?

My work usually pays for my accommodation as I travel. Ordinarily, I stay in the cheapest possible place (e.g. backpackers), but sometimes I stay in more expensive places even though it's not strictly necessary. There'll be some justifications, e.g. the conference will be more convenient to get to; I didn't have time to optimize my bookings; I don't want to make my lab look like it's "cheap"; to avoid staying in mixed-gender accommodation. **Question**: Is unnecessarily staying in a more expensive hotel and charging it to my employer a sin? Depending on how you look at it, this resembles "theft"---I could send a smaller bill to my employer by staying at lower quality accommodation. My employer is not going to get grumpy at me (usually others stay in nicer accommodation than I require).