Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $73.97   13.867%   $84.2274   $84.23   $84.25   $84.20  
  Purchase #2 $52.53   8.794%   $57.1495   $57.15   $57.15   $57.10  
  Purchase #3 $198.84   10.439%   $219.5969   $219.60   $219.60   $219.60  
  Purchase #4 $210.48   3.779%   $218.434   $218.43   $218.45   $218.40  
  Purchase #5 $98.26   6.450%   $104.5978   $104.60   $104.60   $104.60  
  Purchase #6 $34.60   8.569%   $37.5649   $37.56   $37.55   $37.60  
  Purchase #7 $61.83   4.430%   $64.5691   $64.57   $64.55   $64.60  
  7 purchase totals = $730.51       $786.1396   $786.14   $786.15   $786.10