Ideally I would use a thin white washer under the base flange of any taps to provide a seal and prevent rotation. Unfortunately suitable washers are not normally supplied with taps so you have to investigate options from plumbers merchants, etc.
Silicone sealant can be used as an alternative, but it tends to get squeezed out when the back nut is tightened.
I would seal, otherwise water from wet hands (or when the basin/bath is being cleaned) can seep onto the floor.
Someone suggested Plumbers' Mait for this once and it seems to work well for me. Probably not what the pros would use though.
Try not to get it all over the bit of the tails where the nut will go though, or else removing the taps a few years down the line could be very tough.
I also find that the holes in some basins are a bit too big for comfort, and the tap can veer off-centre as the back nut is tightened, sometimes leaving a slightly visible gap at one edge. To help keep it central, I made a ring out of an offcut of 6mm earth wire and slid this over the tap tail before putting the nut on. And no, I'm not going around fitting basin taps to baths etc.
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