What spacers to use for XT crankset when paired with 86.5mm bottom bracket

I am changing my setup from 2-by to 1-by on my gravel bike and got new Shimano XT cranks. My bottom bracket has a shell width of 86.5mm so it is 2mm narrower than the XT cranks.
That means I have to fill the 2 extra millimeters with spacers.
Is there a consensus whether it is advisable to use spacers for the bottom bracket to increase the shell width or to use spacers on the crank spindle and leave the bottom bracket the way it is.
Would love some advice.
Thanks.