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 $28.87   6.484%   $30.7419   $30.74   $30.75   $30.70  
  Purchase #2 $77.28   10.786%   $85.6154   $85.62   $85.60   $85.60  
  Purchase #3 $113.35   4.615%   $118.5811   $118.58   $118.60   $118.60  
  Purchase #4 $107.81   12.302%   $121.0728   $121.07   $121.05   $121.10  
  Purchase #5 $35.84   4.504%   $37.4542   $37.45   $37.45   $37.50  
  Purchase #6 $153.74   3.144%   $158.5736   $158.57   $158.55   $158.60  
  Purchase #7 $93.52   3.222%   $96.5332   $96.53   $96.55   $96.50  
  7 purchase totals = $610.41       $648.5722   $648.56   $648.55   $648.60