You should always check the gap. I recently installed some new Motorcraft ones from the dealer that were for the Focus and didn't check the gap. The car was running very bad so I took them out and checked and they were something like .040 when they should have been .054. Even once fixed they still ran like garbage and I ended up going back to my old Motorcraft plugs with 80,000km on them and they are gapped at .060 and work great.
go to a parts store, and get a gap tool they are only about $1 and then you slide the outside ring in the gap untill you reach the desired gap, you should gap them at about .055 to start and then if its not running right after that then a little bigger untill its right