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 $13.91   11.372%   $15.4918   $15.49   $15.50   $15.50  
  Purchase #2 $103.66   6.115%   $109.9988   $110.00   $110.00   $110.00  
  Purchase #3 $80.40   10.525%   $88.8621   $88.86   $88.85   $88.90  
  Purchase #4 $16.58   9.291%   $18.1204   $18.12   $18.10   $18.10  
  Purchase #5 $34.37   11.113%   $38.1895   $38.19   $38.20   $38.20  
  Purchase #6 $179.63   6.728%   $191.7155   $191.72   $191.70   $191.70  
  Purchase #7 $172.26   10.213%   $189.8529   $189.85   $189.85   $189.90  
  7 purchase totals = $600.81       $652.2310   $652.23   $652.20   $652.30