Fiat > BTC > Alt coin
The conversion does matter but yes, only for the 2 currencies being exchanged.
If I buy BTC at an all time low with fiat, I've received an incredible value. If BTC then jumps up 25% and I use that to purchase another Alt-Coin at a desirable entry point, I've made the absolute most of my investment. If, however, I purchase BTC with fiat and then BTC drops 50% in value, and I then buy into an alt-coin during a climb, I've essentially wasted a large chunk of the fiat potential for when I'd eventually cash out.
IF you hold and make massive gains, this later approach could very well be worth it. An example would be getting into an alt-coin at a rate that would facilitate 2000% gains that even a widely swinging BTC transition couldn't uphold.
All i'm saying is do your own due diligence and dont just invest/convert without accountability