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 $32.34   13.233%   $36.6196   $36.62   $36.60   $36.60  
  Purchase #2 $71.41   9.430%   $78.144   $78.14   $78.15   $78.10  
  Purchase #3 $64.46   5.794%   $68.1948   $68.19   $68.20   $68.20  
  Purchase #4 $211.77   13.495%   $240.3484   $240.35   $240.35   $240.30  
  Purchase #5 $219.83   8.242%   $237.9484   $237.95   $237.95   $237.90  
  Purchase #6 $216.94   6.226%   $230.4467   $230.45   $230.45   $230.40  
  Purchase #7 $125.55   3.245%   $129.6241   $129.62   $129.60   $129.60  
  7 purchase totals = $942.30       $1,021.3260   $1,021.32   $1,021.30   $1,021.10