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 $238.81   3.235%   $246.5355   $246.54   $246.55   $246.50  
  Purchase #2 $83.75   8.653%   $90.9969   $91.00   $91.00   $91.00  
  Purchase #3 $138.27   3.828%   $143.563   $143.56   $143.55   $143.60  
  Purchase #4 $94.75   9.172%   $103.4405   $103.44   $103.45   $103.40  
  Purchase #5 $23.34   9.387%   $25.5309   $25.53   $25.55   $25.50  
  Purchase #6 $167.31   8.223%   $181.0679   $181.07   $181.05   $181.10  
  Purchase #7 $16.42   6.420%   $17.4742   $17.47   $17.45   $17.50  
  7 purchase totals = $762.65       $808.6089   $808.61   $808.60   $808.60