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 $129.64   12.853%   $146.3026   $146.30   $146.30   $146.30  
  Purchase #2 $93.25   4.623%   $97.5609   $97.56   $97.55   $97.60  
  Purchase #3 $242.71   6.138%   $257.6075   $257.61   $257.60   $257.60  
  Purchase #4 $120.33   10.492%   $132.955   $132.96   $132.95   $133.00  
  Purchase #5 $79.83   6.823%   $85.2768   $85.28   $85.30   $85.30  
  Purchase #6 $129.68   11.574%   $144.6892   $144.69   $144.70   $144.70  
  Purchase #7 $117.68   13.864%   $133.9952   $134.00   $134.00   $134.00  
  7 purchase totals = $913.12       $998.3872   $998.40   $998.40   $998.50