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 $20.97   7.272%   $22.4949   $22.49   $22.50   $22.50  
  Purchase #2 $35.65   3.149%   $36.7726   $36.77   $36.75   $36.80  
  Purchase #3 $138.94   9.254%   $151.7975   $151.80   $151.80   $151.80  
  Purchase #4 $146.40   3.664%   $151.7641   $151.76   $151.75   $151.80  
  Purchase #5 $45.45   13.478%   $51.5758   $51.58   $51.60   $51.60  
  Purchase #6 $46.81   10.364%   $51.6614   $51.66   $51.65   $51.70  
  Purchase #7 $74.65   4.261%   $77.8308   $77.83   $77.85   $77.80  
  7 purchase totals = $508.87       $543.8971   $543.89   $543.90   $544.00