Depends what you mean by washing.
I wash plastic and resin miniatures in detergent and water to get rid of mould release agent. And it is compulsory with resin models, so don't skip it.
For example if you've got a forgeworld kit, you will have to wash it. I recommend a mix of fairy liquid and warm orphans tears (it is an expensive GW kit after all).
Or do you mean ink washes?
If you are layering washes are a handy blending tool.
If you are dry and wet brushing, wash with ink and then detail is a good approach.
If you are an NMM'er then ink washes are of limited use to you, as your shading differential is not going to be helped by washes.
And for standard paint it in flat colours, wash and highlight washing if fine.
