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 $123.93   4.670%   $129.7175   $129.72   $129.70   $129.70  
  Purchase #2 $246.62   7.636%   $265.4519   $265.45   $265.45   $265.50  
  Purchase #3 $8.50   10.345%   $9.3793   $9.38   $9.40   $9.40  
  Purchase #4 $9.38   9.862%   $10.3051   $10.31   $10.30   $10.30  
  Purchase #5 $100.60   9.680%   $110.3381   $110.34   $110.35   $110.30  
  Purchase #6 $41.25   3.274%   $42.6005   $42.60   $42.60   $42.60  
  Purchase #7 $63.37   9.743%   $69.5441   $69.54   $69.55   $69.50  
  7 purchase totals = $593.65       $637.3365   $637.34   $637.35   $637.30