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 $244.69   4.865%   $256.5942   $256.59   $256.60   $256.60  
  Purchase #2 $112.63   8.194%   $121.8589   $121.86   $121.85   $121.90  
  Purchase #3 $65.87   7.616%   $70.8867   $70.89   $70.90   $70.90  
  Purchase #4 $132.35   7.230%   $141.9189   $141.92   $141.90   $141.90  
  Purchase #5 $69.55   7.347%   $74.6598   $74.66   $74.65   $74.70  
  Purchase #6 $66.31   12.303%   $74.4681   $74.47   $74.45   $74.50  
  Purchase #7 $237.44   10.471%   $262.3023   $262.30   $262.30   $262.30  
  7 purchase totals = $928.84       $1,002.6889   $1,002.69   $1,002.65   $1,002.80