Is there any simple rule of thumb as to when to trade against USD and BTC?
Maybe always trade against USD unless BTC is in Bull Mode /?
Here's my strategy, when I want more bitcoin i trade my alts against bitcoin, sometimes I might be at loss in regards to the USD value but my focus for that trade isn't for USD value gains instead to accumulate more bitcoin. Now when I want gains specifically for USD value, i trade against USD.
Generally I prefer trading against BTC as it has it's advantage, you gain more USD value (in bull run) or accumulate more bitcoin (in bear market). Trading against USD should be done if your intentions are to cashout your profits immediately.