I agree and disagree with the above posts.
I do think that the hot should always be on the left (in the United States of America) when facing the handles. Not until last week did I ever find a situation where the handles would be different once one is in the tub and facing the handles. The tub this past week was an island tub up on a plateau in the bathroom (with a view of forever), so when walking up to the tub, the hot was correct. When in the tub, however, the hot was incorrect. So it does present a quandary.
If, as one previous poster stated, the hot should be on the left “when in the tub,” that doesn’t take into account the fact that tub users are mostly women, and for some strange reason they seem to like to fill up the tub before getting in. So one would have to step into the tub, turn the faucet on, and step out while the tub fills up. I find that to be impractical.
On the other hand, if they are correct while outside the tub, but incorrect when in the tub, then after relaxing for 30 minutes or so in a hot tub in the company of Mr Rummy Eggnog, if one wanted to stay a little longer, but the water was too hot or too cold, then one could turn on the wrong faucet.
It could get really interesting if she sends her 10-year-old son in to fill up the tub for her.
There’s just all sorts of scenarios, so all I did was point out to my Clients that there were two ways to look at it and to decide which way they wanted to go. They thanked me for the caution.