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 $18.77   9.680%   $20.5869   $20.59   $20.60   $20.60  
  Purchase #2 $72.26   10.741%   $80.0214   $80.02   $80.00   $80.00  
  Purchase #3 $128.59   6.591%   $137.0654   $137.07   $137.05   $137.10  
  Purchase #4 $235.43   3.212%   $242.992   $242.99   $243.00   $243.00  
  Purchase #5 $161.87   5.552%   $170.857   $170.86   $170.85   $170.90  
  Purchase #6 $48.33   7.589%   $51.9978   $52.00   $52.00   $52.00  
  Purchase #7 $134.52   4.475%   $140.5398   $140.54   $140.55   $140.50  
  7 purchase totals = $799.77       $844.0603   $844.07   $844.05   $844.10