I think it depends on the shoe. I see lots of men wearing Cole Haan pinch tassel loafers with a suit, which to me seems totally inappropriate; it looks like they stuck some tassels on a pair of penny loafers, and the look is too casual:
But I think it's more than appropriate to wear AE or Alden tassel loafers with a suit:
I'd be interested in hearing more opinions on the topic.
Tassel loafers with dark suits are a very American tradition, even if not universally preferred by everyone. Most people would agree that the practice adds just a touch of reduced formality in comparison to a lace-up shoe. I normally save my tassel loafers for sport coat days, but will wear them with suits, even dark suits, on occasion. I do agree with vitriol that this option is reserved for traditional tassel loafers -- not tassels added to penny loafers, etc.
I wear Alden/Polo tassels almost exclusively with suits. I love the look and promise you it is totally appropriate.
When I shift down to slacks and spoartcoats, I switch to Alden LHSs.
Anything more casual than that, and I head for my Rockport Perths.
In the tassel world, I'd stay away from some of the non traditional versions (see the CH above). But when you look at the black, cigar, and #8 Cordovan Aldens, or the brown cordovan Polo tassels, along with the brown suede and tan calf Aldens, you can own a pair for any season and any day of the week!
I wear my BB/Alden tassels with gray or navy suits on a regular basis. I agree that they are a touch more informal than balmorals or bluchers, but think they are more than acceptable. Frankly, the shoe selection of many with suits is deplorable and even penny loafers would look better than most of the square toe or skecher type shoes I see.