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 $244.92   12.620%   $275.8289   $275.83   $275.85   $275.80  
  Purchase #2 $100.52   13.346%   $113.9354   $113.94   $113.95   $113.90  
  Purchase #3 $173.95   5.705%   $183.8738   $183.87   $183.85   $183.90  
  Purchase #4 $67.56   8.771%   $73.4857   $73.49   $73.50   $73.50  
  Purchase #5 $41.27   10.371%   $45.5501   $45.55   $45.55   $45.60  
  Purchase #6 $199.61   13.823%   $227.2021   $227.20   $227.20   $227.20  
  Purchase #7 $6.92   12.845%   $7.8089   $7.81   $7.80   $7.80  
  7 purchase totals = $834.75       $927.6849   $927.69   $927.70   $927.70