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 $36.98   9.798%   $40.6033   $40.60   $40.60   $40.60  
  Purchase #2 $226.45   10.844%   $251.0062   $251.01   $251.00   $251.00  
  Purchase #3 $102.67   13.558%   $116.59   $116.59   $116.60   $116.60  
  Purchase #4 $49.74   13.816%   $56.6121   $56.61   $56.60   $56.60  
  Purchase #5 $61.74   9.291%   $67.4763   $67.48   $67.50   $67.50  
  Purchase #6 $10.28   13.692%   $11.6875   $11.69   $11.70   $11.70  
  Purchase #7 $245.96   4.755%   $257.6554   $257.66   $257.65   $257.70  
  7 purchase totals = $733.82       $801.6308   $801.64   $801.65   $801.70