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 $26.89   11.840%   $30.0738   $30.07   $30.05   $30.10  
  Purchase #2 $186.80   13.568%   $212.145   $212.15   $212.15   $212.10  
  Purchase #3 $196.71   13.334%   $222.9393   $222.94   $222.95   $222.90  
  Purchase #4 $139.84   8.508%   $151.7376   $151.74   $151.75   $151.70  
  Purchase #5 $157.55   11.111%   $175.0554   $175.06   $175.05   $175.10  
  Purchase #6 $103.66   8.401%   $112.3685   $112.37   $112.35   $112.40  
  Purchase #7 $63.28   5.301%   $66.6345   $66.63   $66.65   $66.60  
  7 purchase totals = $874.73       $970.9541   $970.96   $970.95   $970.90