Dipping your brush into the can will introduce debris which will haunt you later. Any sludge from exposure to air or other impurieties will announce themselves on your varnished surface.

Just the way it is.

A brand new, never-before-opened can ought not need straining, but once the lid's off the stuff begins to acquire other bits that will frustrate you later on if not dealt with before you apply the fluid to your project.

Dipping your brush into that fresh can will contaminate the product. Maybe not with the first dip, but every one after will, rest assured.

As with paints it's 'best practice' to decant enough product for your immediate purpose into a clean container, from which it will be taken prior to application.

The remaining product left in the can ought to be protected from further exposure to air by whatever means you're prepared to muster.* Otherwise whatever air's replaced the volume of product removed will combine with the varnish to produce a skin. Once that's formed it's nearly impossible to get out 100%. Those hidden bits of cured varnish are what straining's going to remove the next time you go to take more out for application.

(* CLC sells a product for this purpose, basically a plastic bag into which you can pour your finishing fluid. This is a 'squeeze-bag' that allows you to remove what you intend to use while the rest stays fresh in the bag, kept away from the air.

Somthing else is a can of inert gas you can use to displace the oxygen in said can before closing the lid. This keeps it away from the fluid left behind, prevents a skin from forming. I think CLC used to sell this stuff too but I can't find it in their current listings of products....)

unless you are really sloppy and/or want a totally professional finish, i think you can dispense with straining.

first, i only use disposable foam brushes for varnish work....and i make sure that the hull i am varnishing is basically clean...so that i wont pull debris back into the can.

second, i do use like spclark mentioned, the platic bag.  it works great.   so i poor some varnish into a disposable plasitic dixie cup, do my varnish and then toss the cupt and the brush after i am done.    i try to use the cups with lines so i can calibrate how much varnish i need to do a coat/section of the boat.   

there was a time before the plastic bag ....where the varnish would skin over...and evn then i did not use a strainer.  i just picked the skin out with a clean stick....and then continued.  so the bag prevents needing to do that.

my varnish jobs look just fine in my view.