If you are looking at convection microwave ovens, you may have run across some with trim kits. These include parts that allow you to place the appliance into your cabinetry. This provides a 'built-in' look. There are several points to consider before choosing to go with this option.
Have you thought about an over-the-range model that includes an exhaust fan? This option helps to maximize space in your kitchen and may be easier to install.
Counter-top placement puts the microwave at a good level for handling food and to monitor the cooking process. If you don't want to sacrifice room on your counter, consider a cart or island that could house the appliance at the same level.
A shelf can be a more versatile option. If the microwave dies, you can easily put another on the shelf without having to tear into your cabinetry. This will allow you to integrate the microwave into your cabinetry.
Still set on a trim kit? Keep in mind that they can be difficult to install - especially if you want a seamless look. If you are a novice at this type of task, consider hiring someone to do it for you. Of course, this will add to the price of your microwave as well as the extra cost for the trim kit.
Also, make sure that the microwave along with the trim kit will fit in the designated space. If the standard kits will not work, you can order custom trim kits to fit pretty much any space.
Undecided on what you want? In that case, consider getting a counter-top model that offers an optional trim kit. If you decide that placement on the counter isn't going to cut it, then you can purchase the trim kit.