I am planning on completely re-doing the landscaping around my house and I would like to replace the rock pebbles with mulch. Should I put down a layer of plastic or landscaping fabric before adding mulch? What are the pros/cons of doing this?
Definitely not plastic - landscape fabric needs to be water/air permeable. The reason fabric's used is to reduce or stop weed germination, particularly when stones are laid over the top. Even that's not a permanent fix though - over time, particles of soil gather in the stones and eventually weed germination can take place in that top layer of stones.
If you're going to use organic origin mulches instead, a layer 2-3 inches deep is necessary to hold back weed germination if you don't use landscape fabric, less if you do. But, adding organic mulches straight onto soil is beneficial in other ways - the mulch will break down over time and enrich the soil beneath, and retain moisture in the soil, but will need topping up probably yearly. It's also easier to hoe over to remove anything which has germinated in the mulch or soil without fabric present. Plus there's the advantage that any perennial plants or bulbs present will be able to increase their size/width in the soil because they're not restricted by being planted through a hole in landscape fabric. Laid over fabric, a mulch will still need topping up, but if you used, say, chunky bark chips, it will last longer. So it really depends on what plants you're growing and why you're using the mulch - if the main reason is to improve the soil (without digging) and retain moisture, then you don't need the fabric. But for weed suppression, fabric works better, although you should be aware that, even with fabric laid, weed seedlings are perfectly capable of germinating in the organic mulch layer on top of the fabric, and obviously harder to remove because you'd need to do it by hand so as not to disrupt the fabric beneath.