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 $27.76   11.223%   $30.8755   $30.88   $30.90   $30.90  
  Purchase #2 $50.65   11.621%   $56.536   $56.54   $56.55   $56.50  
  Purchase #3 $142.79   9.865%   $156.8762   $156.88   $156.90   $156.90  
  Purchase #4 $132.87   12.348%   $149.2768   $149.28   $149.30   $149.30  
  Purchase #5 $113.33   3.560%   $117.3645   $117.36   $117.35   $117.40  
  Purchase #6 $96.93   11.742%   $108.3115   $108.31   $108.30   $108.30  
  Purchase #7 $49.55   4.380%   $51.7203   $51.72   $51.70   $51.70  
  7 purchase totals = $613.88       $670.9608   $670.97   $671.00   $671.00