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 $21.27   5.419%   $22.4226   $22.42   $22.40   $22.40  
  Purchase #2 $141.54   4.400%   $147.7678   $147.77   $147.75   $147.80  
  Purchase #3 $177.34   12.450%   $199.4188   $199.42   $199.40   $199.40  
  Purchase #4 $7.75   8.262%   $8.3903   $8.39   $8.40   $8.40  
  Purchase #5 $52.95   11.532%   $59.0562   $59.06   $59.05   $59.10  
  Purchase #6 $135.87   6.842%   $145.1662   $145.17   $145.15   $145.20  
  Purchase #7 $96.50   11.497%   $107.5946   $107.59   $107.60   $107.60  
  7 purchase totals = $633.22       $689.8165   $689.82   $689.75   $689.90