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 $128.46   11.490%   $143.2201   $143.22   $143.20   $143.20  
  Purchase #2 $70.55   12.712%   $79.5183   $79.52   $79.50   $79.50  
  Purchase #3 $183.36   4.808%   $192.1759   $192.18   $192.20   $192.20  
  Purchase #4 $110.30   4.830%   $115.6275   $115.63   $115.65   $115.60  
  Purchase #5 $124.53   6.434%   $132.5423   $132.54   $132.55   $132.50  
  Purchase #6 $93.95   9.866%   $103.2191   $103.22   $103.20   $103.20  
  Purchase #7 $141.63   8.636%   $153.8612   $153.86   $153.85   $153.90  
  7 purchase totals = $852.78       $920.1644   $920.17   $920.15   $920.10