Painting cabinets

No, you don't need to put a sealer on it, you can if you like the look or if you're worried that the latex enamel won't hold up to cleaning it.

all cabinets should be sealed

You can add a clear coat to be a little more durable but if you have good coverage and you are happy with it the enamel is good enough

I would as kitchen cupboard seem to need washing often

I painted (using Benjamin Moore Advanced latex paint)my cabinets last year and did not put a sealer on and they still look great. If your paint was a good quality paint, you shouldn't need to seal. If you want to put the time and effort in, no harm done.

Because they are in your kitchen, I would apply the sealer of your choice. If they were anywhere else in your house I would say it's not necessary.

The enamel alone should be good enough.