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 $19.78   3.137%   $20.4005   $20.40   $20.40   $20.40  
  Purchase #2 $145.82   7.795%   $157.1867   $157.19   $157.20   $157.20  
  Purchase #3 $47.42   4.534%   $49.57   $49.57   $49.55   $49.60  
  Purchase #4 $201.58   5.178%   $212.0178   $212.02   $212.00   $212.00  
  Purchase #5 $105.63   3.290%   $109.1052   $109.11   $109.10   $109.10  
  Purchase #6 $158.93   12.399%   $178.6357   $178.64   $178.65   $178.60  
  Purchase #7 $135.56   6.141%   $143.8847   $143.88   $143.90   $143.90  
  7 purchase totals = $814.72       $870.8006   $870.81   $870.80   $870.80