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 $249.31   9.450%   $272.8698   $272.87   $272.85   $272.90  
  Purchase #2 $71.54   7.141%   $76.6487   $76.65   $76.65   $76.60  
  Purchase #3 $240.76   12.143%   $269.9955   $270.00   $270.00   $270.00  
  Purchase #4 $182.82   13.576%   $207.6396   $207.64   $207.65   $207.60  
  Purchase #5 $113.62   11.818%   $127.0476   $127.05   $127.05   $127.00  
  Purchase #6 $52.55   13.413%   $59.5985   $59.60   $59.60   $59.60  
  Purchase #7 $67.99   7.663%   $73.2001   $73.20   $73.20   $73.20  
  7 purchase totals = $978.59       $1,086.9998   $1,087.01   $1,087.00   $1,086.90