If your fiance has a poor driving record, you can expect it to affect your car insurance premium after you get married. Your car insurance policy covers you, your spouse, any other named insured under the policy, and any licensed driver in your household. If any of those people have a bad driving record, it will affect your car insurance rates.
Of course, insurers consider marital status when calculating risk, so the very act of tying the knot may improve your future spouse's risk profile. But if his or her driving record is really bad, you may want to consider additional strategies to stave off a hike in your premium.
If you arrange to purchase all of your insurance policies from one company, you may benefit from multiple policy discounts. Additionally, in some states, your future spouse can take driver safety courses to improve his or her driving record. Further, with a combined income, you might be in a position to raise your deductibles to keep your car insurance premium down.
Your fiance may also qualify for a low mileage discount by using public transportation for his or her commute to work. If all else fails, you may be able to use a "named-driver exclusion" clause after you're married to exclude your spouse from your car insurance policy.