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 $112.42   10.776%   $124.5344   $124.53   $124.55   $124.50  
  Purchase #2 $120.68   13.245%   $136.6641   $136.66   $136.65   $136.70  
  Purchase #3 $76.31   8.381%   $82.7055   $82.71   $82.70   $82.70  
  Purchase #4 $227.82   8.622%   $247.4626   $247.46   $247.45   $247.50  
  Purchase #5 $24.86   7.158%   $26.6395   $26.64   $26.65   $26.60  
  Purchase #6 $196.89   9.774%   $216.134   $216.13   $216.15   $216.10  
  Purchase #7 $154.32   8.230%   $167.0205   $167.02   $167.00   $167.00  
  7 purchase totals = $913.30       $1,001.1606   $1,001.15   $1,001.15   $1,001.10