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 $18.38   7.857%   $19.8241   $19.82   $19.80   $19.80  
  Purchase #2 $175.84   8.771%   $191.2629   $191.26   $191.25   $191.30  
  Purchase #3 $35.70   12.475%   $40.1536   $40.15   $40.15   $40.20  
  Purchase #4 $105.70   13.855%   $120.3447   $120.34   $120.35   $120.30  
  Purchase #5 $191.73   12.304%   $215.3205   $215.32   $215.30   $215.30  
  Purchase #6 $210.83   9.294%   $230.4245   $230.42   $230.40   $230.40  
  Purchase #7 $111.57   6.218%   $118.5074   $118.51   $118.50   $118.50  
  7 purchase totals = $849.75       $935.8377   $935.82   $935.75   $935.80