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 $81.38   8.873%   $88.6008   $88.60   $88.60   $88.60  
  Purchase #2 $228.76   10.743%   $253.3357   $253.34   $253.35   $253.30  
  Purchase #3 $30.80   7.209%   $33.0204   $33.02   $33.00   $33.00  
  Purchase #4 $207.65   11.579%   $231.6938   $231.69   $231.70   $231.70  
  Purchase #5 $63.91   10.215%   $70.4384   $70.44   $70.45   $70.40  
  Purchase #6 $166.81   3.688%   $172.962   $172.96   $172.95   $173.00  
  Purchase #7 $78.69   13.826%   $89.5697   $89.57   $89.55   $89.60  
  7 purchase totals = $858.00       $939.6208   $939.62   $939.60   $939.60