You can use eyeballs, regessed eyeballs if the slope is not to great. You can also get cans for sloped cieling, they will be adjustable to let them point down. However they tend to be larger, ex 6" may be closer to 8". As for layout the customer is always right. If you go over the couch , I prefer to keep them center or slightly back to allow for reading and such. I prefer to use reflector lamps instead of par lamps. You will feel like a french fry under the Pars.
Also be sure to give the dimming option, for those cold winter nights.
Choose your customers, don't let them choose you.