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 $82.45   13.142%   $93.2856   $93.29   $93.30   $93.30  
  Purchase #2 $131.43   12.555%   $147.931   $147.93   $147.95   $147.90  
  Purchase #3 $61.53   12.312%   $69.1056   $69.11   $69.10   $69.10  
  Purchase #4 $146.72   11.276%   $163.2641   $163.26   $163.25   $163.30  
  Purchase #5 $144.42   10.268%   $159.249   $159.25   $159.25   $159.20  
  Purchase #6 $77.67   4.753%   $81.3617   $81.36   $81.35   $81.40  
  Purchase #7 $234.88   13.149%   $265.7644   $265.76   $265.75   $265.80  
  7 purchase totals = $879.10       $979.9614   $979.96   $979.95   $980.00