Having bad breath is a major turn off for many partners out there. Having good breath is something you should want to have around people in general. It is especially important for a French kiss as you will be exchanging saliva with your partner. Try to not to eat smelly things before a kiss such as garlic or onions. Gum and mints help, but can only do so much. In the event of having smelly food, or in the event of having food that leaves a residue in your mouth, it is best to brush your teeth if you can. If you can't bush your teeth, try drinking lots of water to clean out your mouth. Then try mints/gum.
This is not necessary for all partners, but until you know your partner's kissing preferences, it is a good thing to take into consideration as most people enjoy the clean palate so that they can focus on the kiss itself.