I do whatever the fabric tells me to do. Some patterns really do not do well with a slanted seam and others do not do well with an angled seam. (And, sometimes I am so close on the amount of fabric I have that I have to use a straight seam in order for it to work!)
In general, I will use the slanted seam because it looks a little better usually.
As far as bulk is concerned, I press my joining seams open and I have never had an issue with bulk.
That being said, I make quilts to be used and loved - not to look at. That would make a difference, too.