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 $131.28   6.231%   $139.4601   $139.46   $139.45   $139.50  
  Purchase #2 $115.81   4.540%   $121.0678   $121.07   $121.05   $121.10  
  Purchase #3 $175.50   13.255%   $198.7625   $198.76   $198.75   $198.80  
  Purchase #4 $197.80   11.473%   $220.4936   $220.49   $220.50   $220.50  
  Purchase #5 $115.99   6.696%   $123.7567   $123.76   $123.75   $123.80  
  Purchase #6 $38.27   4.375%   $39.9443   $39.94   $39.95   $39.90  
  Purchase #7 $93.70   5.783%   $99.1187   $99.12   $99.10   $99.10  
  7 purchase totals = $868.35       $942.6037   $942.60   $942.55   $942.70