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 $109.81   3.430%   $113.5765   $113.58   $113.60   $113.60  
  Purchase #2 $72.28   6.283%   $76.8214   $76.82   $76.80   $76.80  
  Purchase #3 $8.98   5.606%   $9.4834   $9.48   $9.50   $9.50  
  Purchase #4 $153.46   12.842%   $173.1673   $173.17   $173.15   $173.20  
  Purchase #5 $15.72   7.453%   $16.8916   $16.89   $16.90   $16.90  
  Purchase #6 $138.51   4.146%   $144.2526   $144.25   $144.25   $144.30  
  Purchase #7 $250.34   11.403%   $278.8863   $278.89   $278.90   $278.90  
  7 purchase totals = $749.10       $813.0791   $813.08   $813.10   $813.20