You should check the specifications for your fridge and freezer, specifically the section that discusses required clearances. If you provided enough clearance, then the heat is likely not an issue as the fridge/freezer is designed to dissipate heat given the specified clearances.
If your clearances are too small then there is a good chance the device will not be able to properly dissipate the heat it produces. This could possibily cause a fire, but the most likely scenario is that the devices will fail early due to components overheating.
Adding ventilation is not necessarily going to help, certainly not from a warranty perspective, but ensuring the correct clearance will.
The other thing to consider is the impact of the heat on your cabinets. Ventilation will help this.
Everything Matthew said is accurate, good answer if you're mounting very close to the ceiling. A ledger is a good way to keep the uppers aligned and support the cabinet weight while installing.
There are several other techniques you should know however.
1) Screws holding the weight should be through a framing member or stile, not through a thin backer panel, they should go into a stud or a horizontal nailer installed between studs for this specific purpose. you may have to remove a strip of sheetrock to install a nailer. Do not screw into just sheetrock, even with anchors or your cabinets will be on the floor before you know it. Don't use drywall screws, get cabinet screws that have a built on washer.
2) Adjacent cabinets need to be clamped together and fastened together through the side stiles with counter sunk screws. This locks them together and assures the meeting stiles are flush and stay flush.
3) Always start in a corner if you have cabinets going down two walls. the corner cabinet is your anchor.
4) Carefully check the level of the floor and use embossing material to fill and sags or dips.
5) Dry fit the lowers, before attaching to the wall, shim the low ones, clamp the adjacent stiles together as you did with the uppers. Screw them to the wall, checking level in both directions (length and perpendicular to wall), then screw the stiles together as above.
6) measure precisely the openings required by your appliances. Give yourself no more than 1/4" over the actual appliance dimension. 1/8" is better. 1/16" on either side.
7) Install the lower kick plate last to cover and shims and gaps.
There are a lot of other tricks of the trade, but go slow, pre-plan and have a little extra man power to handle the cabinets.
Google up "This Old House" and watch some of the many videos on this topic.
Best Answer
You need something like these Heavy Duty Cabinet Hangers:
components http://www.screwmaster.co.uk/ekmps/shops/screwmaster/images/pair-of-heavy-duty-cabinet-hangers-260kg-load-capacity-for-kitchen-wall-units-57-p.jpg
The metal plates at the top screw to the wall, the hook like things in the middle screw to the inside of the cabinet and the plastic covers them up.
in place http://www.screwmaster.co.uk/ekmps/shops/screwmaster/images/pair-of-heavy-duty-cabinet-hangers-260kg-load-capacity-for-kitchen-wall-units-%5B3%5D-57-p.jpg
You do need to cut a hole in the backing for the hook, but the plastic will hide these as well.