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 $228.70   4.740%   $239.5404   $239.54   $239.55   $239.50  
  Purchase #2 $84.28   3.584%   $87.3006   $87.30   $87.30   $87.30  
  Purchase #3 $182.63   12.308%   $205.1081   $205.11   $205.10   $205.10  
  Purchase #4 $31.59   3.590%   $32.7241   $32.72   $32.70   $32.70  
  Purchase #5 $48.28   12.386%   $54.26   $54.26   $54.25   $54.30  
  Purchase #6 $236.59   9.191%   $258.335   $258.34   $258.35   $258.30  
  Purchase #7 $78.94   4.473%   $82.471   $82.47   $82.45   $82.50  
  7 purchase totals = $891.01       $959.7392   $959.74   $959.70   $959.70