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 $9.86   5.489%   $10.4012   $10.40   $10.40   $10.40  
  Purchase #2 $79.67   6.206%   $84.6143   $84.61   $84.60   $84.60  
  Purchase #3 $91.51   8.758%   $99.5244   $99.52   $99.50   $99.50  
  Purchase #4 $47.80   7.580%   $51.4232   $51.42   $51.40   $51.40  
  Purchase #5 $105.48   3.842%   $109.5325   $109.53   $109.55   $109.50  
  Purchase #6 $142.53   5.179%   $149.9116   $149.91   $149.90   $149.90  
  Purchase #7 $233.32   13.538%   $264.9069   $264.91   $264.90   $264.90  
  7 purchase totals = $710.17       $770.3141   $770.30   $770.25   $770.20