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 $236.59   6.222%   $251.3106   $251.31   $251.30   $251.30  
  Purchase #2 $199.88   9.479%   $218.8266   $218.83   $218.85   $218.80  
  Purchase #3 $12.67   6.649%   $13.5124   $13.51   $13.50   $13.50  
  Purchase #4 $7.83   7.468%   $8.4147   $8.41   $8.40   $8.40  
  Purchase #5 $207.66   9.294%   $226.9599   $226.96   $226.95   $227.00  
  Purchase #6 $125.81   10.592%   $139.1358   $139.14   $139.15   $139.10  
  Purchase #7 $84.67   4.409%   $88.4031   $88.40   $88.40   $88.40  
  7 purchase totals = $875.11       $946.5631   $946.56   $946.55   $946.50