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 $13.31   11.590%   $14.8526   $14.85   $14.85   $14.90  
  Purchase #2 $15.33   7.676%   $16.5067   $16.51   $16.50   $16.50  
  Purchase #3 $190.38   12.354%   $213.8995   $213.90   $213.90   $213.90  
  Purchase #4 $148.94   10.270%   $164.2361   $164.24   $164.25   $164.20  
  Purchase #5 $102.61   7.308%   $110.1087   $110.11   $110.10   $110.10  
  Purchase #6 $246.72   12.510%   $277.5847   $277.58   $277.60   $277.60  
  Purchase #7 $39.34   8.695%   $42.7606   $42.76   $42.75   $42.80  
  7 purchase totals = $756.63       $839.9489   $839.95   $839.95   $840.00