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 $62.43   6.659%   $66.5872   $66.59   $66.60   $66.60  
  Purchase #2 $15.47   13.191%   $17.5106   $17.51   $17.50   $17.50  
  Purchase #3 $248.96   13.746%   $283.182   $283.18   $283.20   $283.20  
  Purchase #4 $204.82   6.781%   $218.7088   $218.71   $218.70   $218.70  
  Purchase #5 $81.98   11.403%   $91.3282   $91.33   $91.35   $91.30  
  Purchase #6 $132.88   11.591%   $148.2821   $148.28   $148.30   $148.30  
  Purchase #7 $107.28   8.804%   $116.7249   $116.72   $116.70   $116.70  
  7 purchase totals = $853.82       $942.3238   $942.32   $942.35   $942.30