I've done that once on the Wheat wash from a few years back. It worked really well. It's kind of up to you on how much you want to taper - I brought in my low straights and told the guy to match the leg opening. So I'd say it's a little wider than a slim fit would have been, the extreme effect of the flare is gone. It's probably one of my favorite pairs of jeans now because it's a more balanced fit for my build. In general, I find slim fit too tapered for my me because I have enormous calves..
Did your tailor taper it from the inseam? I want to taper a pair of slim bootcuts but want to keep the selvedge outseam. Not sure if tailoring it from the inseam would throw off the shape of the lower leg.