I start with my swatch, so I don’t have surprises after I have invested my money and time in the project.
Acrylics and other synthetic fibres need very little; it will rid the item of dust and grease from handling. Please keep in mind that “heat” will change the composition of the acrylic. i.e. from a clothes dryer.
Wool benefits from ‘wet blocking’ for the same reasons, but in addition, it also washes out the chemicals that are used in spinning the fleece. This varies from mill to mill, spinner to spinner.
A woollen item also can be manipulated for fitting purposes or look. i.e. felting/fulling.
Cotton and cotton mixes also may produce surprises.
My recommendation is to treat the swatch as you would the finished item. If using an automatic washing machine and dryer, do so to the swatch. I usually ‘air dry’ my garments.