So, I have previously been doing bicep curls by holding the dumbell across my front and then lifting them like that. But, I've seen other people hold the dumbell at their sides and then twist it as they're bringing it up till it's across their front. Someone at the rec center told me that if you twist it it works all of your biceps as opposed to if you just keep it in one position, which works only one bicep. Is this true?

It really is not going to make any significant difference. I prefer twisting it as I come up though.