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 $119.54   12.352%   $134.3056   $134.31   $134.30   $134.30  
  Purchase #2 $66.32   12.547%   $74.6412   $74.64   $74.65   $74.60  
  Purchase #3 $76.45   8.545%   $82.9827   $82.98   $83.00   $83.00  
  Purchase #4 $19.99   10.287%   $22.0464   $22.05   $22.05   $22.00  
  Purchase #5 $126.52   9.752%   $138.8582   $138.86   $138.85   $138.90  
  Purchase #6 $116.97   6.284%   $124.3204   $124.32   $124.30   $124.30  
  Purchase #7 $238.98   11.593%   $266.685   $266.69   $266.70   $266.70  
  7 purchase totals = $764.77       $843.8395   $843.85   $843.85   $843.80