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 $9.60   5.724%   $10.1495   $10.15   $10.15   $10.10  
  Purchase #2 $44.77   12.471%   $50.3533   $50.35   $50.35   $50.40  
  Purchase #3 $204.71   8.339%   $221.7808   $221.78   $221.80   $221.80  
  Purchase #4 $7.67   13.870%   $8.7338   $8.73   $8.75   $8.70  
  Purchase #5 $132.40   4.527%   $138.3937   $138.39   $138.40   $138.40  
  Purchase #6 $20.56   7.763%   $22.1561   $22.16   $22.15   $22.20  
  Purchase #7 $150.25   10.184%   $165.5515   $165.55   $165.55   $165.60  
  7 purchase totals = $569.96       $617.1187   $617.11   $617.15   $617.20