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 $34.53   13.815%   $39.3003   $39.30   $39.30   $39.30  
  Purchase #2 $186.81   13.354%   $211.7566   $211.76   $211.75   $211.80  
  Purchase #3 $16.52   6.547%   $17.6016   $17.60   $17.60   $17.60  
  Purchase #4 $240.35   13.546%   $272.9078   $272.91   $272.90   $272.90  
  Purchase #5 $58.85   11.394%   $65.5554   $65.56   $65.55   $65.60  
  Purchase #6 $43.83   3.509%   $45.368   $45.37   $45.35   $45.40  
  Purchase #7 $10.55   6.354%   $11.2203   $11.22   $11.20   $11.20  
  7 purchase totals = $591.44       $663.7100   $663.72   $663.65   $663.80