I didn't know this is now a common practice. Wow! Not something that excites me in a good way.
Now, I have heard of some taking androgens which normally have poor oral bioavailability (those not 17 alpha-alkylated) to increase oral bioavailability. First let me say this, when I did look in to the available data, there was both supportive and unsupportive data for the idea that consumption would increase oral bioavailability. The second problem was the assumption that grapefruit juice is an absolute selective inhibitor of a particular cyp isoenzyme, as this is not the case. This creates the potential for people to unwittingly assume that taking a particular drug or supplement is safe, when in fact, doing so may result in dramatically larger blood levels, resulting in toxicity.
Now in respect to people taking grapefruit juice with 17 alpha-alkylated androgens, this is not something I'd suggest. First, the oral bioavailability of such compounds is good enough to where this shouldn't be a concern. Secondly, decreasing hepatic metabolism of these compounds will likely result in greater hepatotoxicity (and increased likelihood of other noted side effects) as you're achieving much higher blood levels than you normally would.
As for the grapefruit juice and things like milk thistle, interestingly enough, they both are inhibitors of the same major isoenzyme of concern when talking about steroids. In other words, you're likely going to end up with higher blood levels than you normally would with milk thistle as well. The benefit however, as compared to grapefruit juice, is that milk has fairly well-documented hepato-protective effects.
In short, I would not use the grapefruit juice and stick to the milk thistle.
In terms of how effective such products are, I often question how necessary they are for responsible and healthy recreational users. Certainly for those who use large dosages for prolonged periods (certain high level amateurs and most professional bodybuilders) I think you could make a good case for use, but generally speaking, I think most will have no problems as the issue of hepatotoxicity is greatly overblown.
I think a question that one might need to ask is this:
If indeed you're getting higher blood levels of the oral steroid with milk thistle, while having this hepato-protective effect, what about addressing other possible adverse effects of having higher blood levels of these steroids?
As an answer, anecdotally at least, it would seem that that use of milk thistle as a prophylaxis isn't a problem.