A few years ago Expedia tried changing our booking to a 45 min connection they rang me to tell me if the airline were offering it then I should take it as they would get me on the next if we didn't make it. I looked at the airlines schedule and saw there was availabilty on one with 2 hours 30 mins, Expedia were very awkward about it but I got them to change in the end.
I have done about 10 connections in US, there are too many factors to go wrong to accept 45 mins. Late departure from UK, Lng Immigration lines etc. 90 mins is really least you should accept, many prefer 2 hours.
Last Oct on way to Vegas we did it in 1 hour and that was a rush, glad our 2nd leg was delayed 30 mins.