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 $8.53   11.130%   $9.4794   $9.48   $9.50   $9.50  
  Purchase #2 $78.93   7.516%   $84.8624   $84.86   $84.85   $84.90  
  Purchase #3 $127.75   4.413%   $133.3876   $133.39   $133.40   $133.40  
  Purchase #4 $52.71   5.344%   $55.5268   $55.53   $55.55   $55.50  
  Purchase #5 $167.41   4.352%   $174.6957   $174.70   $174.70   $174.70  
  Purchase #6 $74.65   9.845%   $81.9993   $82.00   $82.00   $82.00  
  Purchase #7 $117.33   8.270%   $127.0332   $127.03   $127.05   $127.00  
  7 purchase totals = $627.31       $666.9844   $666.99   $667.05   $667.00