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 $10.76   11.386%   $11.9851   $11.99   $12.00   $12.00  
  Purchase #2 $15.87   9.649%   $17.4013   $17.40   $17.40   $17.40  
  Purchase #3 $172.78   5.285%   $181.9114   $181.91   $181.90   $181.90  
  Purchase #4 $32.48   13.867%   $36.984   $36.98   $37.00   $37.00  
  Purchase #5 $210.54   6.700%   $224.6462   $224.65   $224.65   $224.60  
  Purchase #6 $37.65   11.829%   $42.1036   $42.10   $42.10   $42.10  
  Purchase #7 $117.45   8.862%   $127.8584   $127.86   $127.85   $127.90  
  7 purchase totals = $597.53       $642.8900   $642.89   $642.90   $642.90