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 $18.97   6.559%   $20.2142   $20.21   $20.20   $20.20  
  Purchase #2 $80.93   12.182%   $90.7889   $90.79   $90.80   $90.80  
  Purchase #3 $18.66   9.513%   $20.4351   $20.44   $20.45   $20.40  
  Purchase #4 $136.91   3.860%   $142.1947   $142.19   $142.20   $142.20  
  Purchase #5 $43.62   5.732%   $46.1203   $46.12   $46.10   $46.10  
  Purchase #6 $70.46   12.415%   $79.2076   $79.21   $79.20   $79.20  
  Purchase #7 $71.81   11.669%   $80.1895   $80.19   $80.20   $80.20  
  7 purchase totals = $441.36       $479.1503   $479.15   $479.15   $479.10