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 $56.51   7.519%   $60.759   $60.76   $60.75   $60.80  
  Purchase #2 $61.25   8.697%   $66.5769   $66.58   $66.60   $66.60  
  Purchase #3 $160.28   5.850%   $169.6564   $169.66   $169.65   $169.70  
  Purchase #4 $61.46   9.133%   $67.0731   $67.07   $67.05   $67.10  
  Purchase #5 $202.74   8.213%   $219.391   $219.39   $219.40   $219.40  
  Purchase #6 $16.74   5.345%   $17.6348   $17.63   $17.65   $17.60  
  Purchase #7 $41.58   9.551%   $45.5513   $45.55   $45.55   $45.60  
  7 purchase totals = $600.56       $646.6425   $646.64   $646.65   $646.80