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 $81.52   9.295%   $89.0973   $89.10   $89.10   $89.10  
  Purchase #2 $11.61   9.535%   $12.717   $12.72   $12.70   $12.70  
  Purchase #3 $97.62   11.592%   $108.9361   $108.94   $108.95   $108.90  
  Purchase #4 $180.59   4.126%   $188.0411   $188.04   $188.05   $188.00  
  Purchase #5 $201.36   13.336%   $228.2134   $228.21   $228.20   $228.20  
  Purchase #6 $20.59   13.726%   $23.4162   $23.42   $23.40   $23.40  
  Purchase #7 $35.98   10.134%   $39.6262   $39.63   $39.65   $39.60  
  7 purchase totals = $629.27       $690.0473   $690.06   $690.05   $689.90