I'd suggest spreading out the proposals, especially if all of the bloggers are within the same niche. It will be obvious to blog readers interested in the niche that the reviews are paid for if every blog is essentially promoting the same product/category of products within the same time period. I would also suggest varying the products if possible (although I don't know anything about the products you are offering) just so 150 blogs aren't reviewing the exact same product, which again might turn off blog readers and could even raise some flags with the search engines as being "sponsored posts" (especially if they all show up at once).
That being said, if you are looking to generate some publicity for a new product launch, it might be a better idea to do some blogger outreach combined with press releases and articles/interviews with major news outlets just so it comes off as more "legitimate" buzz instead of simply getting a bunch of bloggers to review the same product at the same time.