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 $42.99   5.580%   $45.3888   $45.39   $45.40   $45.40  
  Purchase #2 $168.32   5.167%   $177.0171   $177.02   $177.00   $177.00  
  Purchase #3 $113.33   13.857%   $129.0341   $129.03   $129.05   $129.00  
  Purchase #4 $170.95   7.186%   $183.2345   $183.23   $183.25   $183.20  
  Purchase #5 $182.36   11.779%   $203.8402   $203.84   $203.85   $203.80  
  Purchase #6 $131.32   8.777%   $142.846   $142.85   $142.85   $142.80  
  Purchase #7 $99.81   8.299%   $108.0932   $108.09   $108.10   $108.10  
  7 purchase totals = $909.08       $989.4539   $989.45   $989.50   $989.30