Allotments 4 All

Is is okay to use grass clippings to mulch my tomatoes? 

Make sure that you have not sprayed the grass with anything. One of the grass sprays contains Clopiralid weed killer, which will ruin your tomatoes if you mulch with the grass clippings.

As above, make sure the grass hasn't been treated with any mosskiller or selective weedkillers. These would still affect other plants.  If, like mine, your grass doesn't get mown as often as it should i'd be wary of grass seeds in the clippings.