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 $217.62   11.287%   $242.1828   $242.18   $242.20   $242.20  
  Purchase #2 $6.49   4.796%   $6.8013   $6.80   $6.80   $6.80  
  Purchase #3 $246.89   5.292%   $259.9554   $259.96   $259.95   $260.00  
  Purchase #4 $138.79   13.199%   $157.1089   $157.11   $157.10   $157.10  
  Purchase #5 $33.67   6.531%   $35.869   $35.87   $35.85   $35.90  
  Purchase #6 $67.95   12.436%   $76.4003   $76.40   $76.40   $76.40  
  Purchase #7 $126.75   5.324%   $133.4982   $133.50   $133.50   $133.50  
  7 purchase totals = $838.16       $911.8159   $911.82   $911.80   $911.90