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 $238.60   6.790%   $254.8009   $254.80   $254.80   $254.80  
  Purchase #2 $179.80   13.560%   $204.1809   $204.18   $204.20   $204.20  
  Purchase #3 $128.41   8.564%   $139.407   $139.41   $139.40   $139.40  
  Purchase #4 $94.84   3.214%   $97.8882   $97.89   $97.90   $97.90  
  Purchase #5 $98.90   12.215%   $110.9806   $110.98   $111.00   $111.00  
  Purchase #6 $13.30   13.268%   $15.0646   $15.06   $15.05   $15.10  
  Purchase #7 $56.31   7.311%   $60.4268   $60.43   $60.45   $60.40  
  7 purchase totals = $810.16       $882.7490   $882.75   $882.80   $882.80