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 $16.82   12.597%   $18.9388   $18.94   $18.95   $18.90  
  Purchase #2 $70.89   13.471%   $80.4396   $80.44   $80.45   $80.40  
  Purchase #3 $104.47   4.439%   $109.1074   $109.11   $109.10   $109.10  
  Purchase #4 $56.69   8.625%   $61.5795   $61.58   $61.60   $61.60  
  Purchase #5 $24.93   11.255%   $27.7359   $27.74   $27.75   $27.70  
  Purchase #6 $160.42   6.125%   $170.2457   $170.25   $170.25   $170.20  
  Purchase #7 $50.49   4.790%   $52.9085   $52.91   $52.90   $52.90  
  7 purchase totals = $484.71       $520.9554   $520.97   $521.00   $520.80