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 $59.84   9.557%   $65.5589   $65.56   $65.55   $65.60  
  Purchase #2 $90.84   12.610%   $102.2949   $102.29   $102.30   $102.30  
  Purchase #3 $230.66   5.781%   $243.9945   $243.99   $244.00   $244.00  
  Purchase #4 $148.93   9.426%   $162.9681   $162.97   $162.95   $163.00  
  Purchase #5 $9.99   13.116%   $11.3003   $11.30   $11.30   $11.30  
  Purchase #6 $177.50   4.414%   $185.3349   $185.33   $185.35   $185.30  
  Purchase #7 $61.42   11.384%   $68.4121   $68.41   $68.40   $68.40  
  7 purchase totals = $779.18       $839.8637   $839.85   $839.85   $839.90