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 $17.30   9.457%   $18.9361   $18.94   $18.95   $18.90  
  Purchase #2 $117.50   7.619%   $126.4523   $126.45   $126.45   $126.50  
  Purchase #3 $124.51   9.646%   $136.5202   $136.52   $136.50   $136.50  
  Purchase #4 $154.94   10.673%   $171.4767   $171.48   $171.50   $171.50  
  Purchase #5 $205.58   6.603%   $219.1544   $219.15   $219.15   $219.20  
  Purchase #6 $16.51   5.607%   $17.4357   $17.44   $17.45   $17.40  
  Purchase #7 $10.33   7.558%   $11.1107   $11.11   $11.10   $11.10  
  7 purchase totals = $646.67       $701.0861   $701.09   $701.10   $701.10