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 $36.52   10.717%   $40.4338   $40.43   $40.45   $40.40  
  Purchase #2 $66.47   5.760%   $70.2987   $70.30   $70.30   $70.30  
  Purchase #3 $235.84   9.330%   $257.8439   $257.84   $257.85   $257.80  
  Purchase #4 $146.31   13.454%   $165.9945   $165.99   $166.00   $166.00  
  Purchase #5 $207.30   11.580%   $231.3053   $231.31   $231.30   $231.30  
  Purchase #6 $115.92   10.169%   $127.7079   $127.71   $127.70   $127.70  
  Purchase #7 $5.69   13.276%   $6.4454   $6.45   $6.45   $6.40  
  7 purchase totals = $814.05       $900.0295   $900.03   $900.05   $899.90