Is it better to water my houseplants before or after spraying bayer products to control spider mites?
If you can water by pouring the water directly on the potting mix, it doesn't matter, you can water anytime.
If your plant is under an overhead irrigation system, then of course you want to water before you spray, or wait until the spray has dried, or you will wash it off of the plant, and the application will be rendered useless for the most part.
Insecticidal soaps only need a few minutes to be effective. A rinse to wash the soap off after a few applications is highly recommended to prevent the buildup of fatty acids on the "business" part of the leaf where gaseous exchange goes on.
Some plants such as african violets do not respond well to soap treatments. Either they have more delicate leaves or less tolerance for a coating.
It does not matter whether you water your plants before or after spraying them with an insecticidal soap. It does help if you rinse the leaves afterward to remove the soapy residue and repeat the treatment at least three times at five to seven day intervals to catch the next generation of pests as they hatch.