For God's sake, YES. Yes you MUST have a complete rain plan.
When planning a wedding reception, it's important to have a rain plan in place. Even if you're getting married during the dry season or in a location that's not known for rain, unexpected weather can still occur. Here are a few reasons why having a rain plan is important:
To ensure the safety of your guests - If it's raining heavily, it can be dangerous for guests to be outside. Having a rain plan in place ensures that your guests will be safe and comfortable throughout the reception.
To protect your vendors - Your vendors, including the catering team, DJ, and photographer, may have sensitive equipment that can be damaged by rain. Having a rain plan in place can help protect their equipment and ensure that they can provide their services without any issues.
To maintain the overall aesthetic of your wedding - If you've spent months planning the perfect outdoor wedding, rain can put a damper on your plans. Having a rain plan in place ensures that you can still achieve the overall aesthetic of your wedding, even if it's moved indoors.
To minimize stress on your wedding day - Rain can cause a lot of stress and anxiety for the couple and their families. Having a rain plan in place can help minimize this stress and ensure that everything runs smoothly on your special day.
In our experience (which is extensive) there tends to be a karmic, Murphy's Law dynamic at work here as well - if you have a detailed, well thought out rain plan, usually, you won't have to use it. However, if your rain plan is one of those "We'll cross that bridge if we come to it." strategies, you can pretty well count on needing a canoe to get down the isle.
Hope for the best, but make plans for the other thing too. This will guarantee you the best, most stress-free wedding day possible.