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 $110.77   9.567%   $121.3674   $121.37   $121.35   $121.40  
  Purchase #2 $199.85   12.281%   $224.3936   $224.39   $224.40   $224.40  
  Purchase #3 $142.87   10.413%   $157.7471   $157.75   $157.75   $157.70  
  Purchase #4 $104.35   10.601%   $115.4121   $115.41   $115.40   $115.40  
  Purchase #5 $100.79   11.245%   $112.1238   $112.12   $112.10   $112.10  
  Purchase #6 $71.54   10.167%   $78.8135   $78.81   $78.80   $78.80  
  Purchase #7 $104.81   11.671%   $117.0424   $117.04   $117.05   $117.00  
  7 purchase totals = $834.98       $926.8999   $926.89   $926.85   $926.80