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 $61.43   8.697%   $66.7726   $66.77   $66.75   $66.80  
  Purchase #2 $128.47   9.732%   $140.9727   $140.97   $140.95   $141.00  
  Purchase #3 $39.42   6.476%   $41.9728   $41.97   $41.95   $42.00  
  Purchase #4 $39.39   12.679%   $44.3843   $44.38   $44.40   $44.40  
  Purchase #5 $202.67   5.540%   $213.8979   $213.90   $213.90   $213.90  
  Purchase #6 $37.83   5.658%   $39.9704   $39.97   $39.95   $40.00  
  Purchase #7 $224.77   10.871%   $249.2047   $249.20   $249.20   $249.20  
  7 purchase totals = $733.98       $797.1754   $797.16   $797.10   $797.30