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 $7.95   5.459%   $8.384   $8.38   $8.40   $8.40  
  Purchase #2 $39.75   10.798%   $44.0422   $44.04   $44.05   $44.00  
  Purchase #3 $108.29   13.207%   $122.5919   $122.59   $122.60   $122.60  
  Purchase #4 $73.31   8.251%   $79.3588   $79.36   $79.35   $79.40  
  Purchase #5 $29.46   4.404%   $30.7574   $30.76   $30.75   $30.80  
  Purchase #6 $7.96   11.677%   $8.8895   $8.89   $8.90   $8.90  
  Purchase #7 $104.49   3.104%   $107.7334   $107.73   $107.75   $107.70  
  7 purchase totals = $371.21       $401.7572   $401.75   $401.80   $401.80