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 $30.80   3.505%   $31.8795   $31.88   $31.90   $31.90  
  Purchase #2 $160.70   12.673%   $181.0655   $181.07   $181.05   $181.10  
  Purchase #3 $145.40   9.117%   $158.6561   $158.66   $158.65   $158.70  
  Purchase #4 $160.83   10.143%   $177.143   $177.14   $177.15   $177.10  
  Purchase #5 $66.56   5.741%   $70.3812   $70.38   $70.40   $70.40  
  Purchase #6 $176.52   6.736%   $188.4104   $188.41   $188.40   $188.40  
  Purchase #7 $85.49   9.381%   $93.5098   $93.51   $93.50   $93.50  
  7 purchase totals = $826.30       $901.0455   $901.05   $901.05   $901.10