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 $28.25   12.854%   $31.8813   $31.88   $31.90   $31.90  
  Purchase #2 $42.31   3.846%   $43.9372   $43.94   $43.95   $43.90  
  Purchase #3 $107.86   13.420%   $122.3348   $122.33   $122.35   $122.30  
  Purchase #4 $116.30   5.222%   $122.3732   $122.37   $122.35   $122.40  
  Purchase #5 $104.31   3.848%   $108.3238   $108.32   $108.30   $108.30  
  Purchase #6 $114.90   5.594%   $121.3275   $121.33   $121.35   $121.30  
  Purchase #7 $193.54   6.556%   $206.2285   $206.23   $206.25   $206.20  
  7 purchase totals = $707.47       $756.4063   $756.40   $756.45   $756.30