Short answer, no, there's generally no point in resizing before submitting to a stock agency.
With most agencies, once the customer chooses to buy a photo (actually a license to use the photo; they don't actually get the copyright) and pays for it, they can download the photo immediately. The agency isn't going to contact you to get the high-resolution version because they already have both the high-res and your permission to license it. Whatever resolution you submit to the agency should be the highest resolution you want to sell.
As far as the photo getting stolen, who are you concerned about? Any reputable agency will make every effort to keep people from stealing the photos from the website; they have just as much of an interest as you do to prevent people from using the photos illegally (i.e., without paying them the licensing fees). If you're concerned about the agency staying the photos, you need to find a different agency.
By submitting lower-resolution images, you're actually only hurting yourself. Royalty-free stock sites usually allow the customer to select the resolution they want; higher-resolution images are higher prices. Pricing for rights-managed images for print usually includes the size of the image on the page; larger images cost more but requires a higher-resolution photo. By submitting a lower-resolution image, you're cutting yourself out of the highest pricing categories.
Disclaimer: I've not submitted or sold stock, but part of my job involves researching stock. This answer is based on that experience.