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 $230.32   4.486%   $240.6522   $240.65   $240.65   $240.70  
  Purchase #2 $65.37   3.663%   $67.7645   $67.76   $67.75   $67.80  
  Purchase #3 $130.51   5.481%   $137.6633   $137.66   $137.65   $137.70  
  Purchase #4 $99.51   5.196%   $104.6805   $104.68   $104.70   $104.70  
  Purchase #5 $17.39   8.419%   $18.8541   $18.85   $18.85   $18.90  
  Purchase #6 $62.40   12.366%   $70.1164   $70.12   $70.10   $70.10  
  Purchase #7 $35.65   11.710%   $39.8246   $39.82   $39.80   $39.80  
  7 purchase totals = $641.15       $679.5556   $679.54   $679.50   $679.70