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 $35.83   13.281%   $40.5886   $40.59   $40.60   $40.60  
  Purchase #2 $131.59   8.622%   $142.9357   $142.94   $142.95   $142.90  
  Purchase #3 $113.61   13.220%   $128.6292   $128.63   $128.65   $128.60  
  Purchase #4 $161.42   6.576%   $172.035   $172.04   $172.05   $172.00  
  Purchase #5 $181.77   7.514%   $195.4282   $195.43   $195.45   $195.40  
  Purchase #6 $225.36   3.832%   $233.9958   $234.00   $234.00   $234.00  
  Purchase #7 $157.27   4.711%   $164.679   $164.68   $164.70   $164.70  
  7 purchase totals = $1,006.85       $1,078.2915   $1,078.31   $1,078.40   $1,078.20