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 $28.96   3.818%   $30.0657   $30.07   $30.05   $30.10  
  Purchase #2 $94.49   8.833%   $102.8363   $102.84   $102.85   $102.80  
  Purchase #3 $19.80   3.873%   $20.5669   $20.57   $20.55   $20.60  
  Purchase #4 $199.30   10.108%   $219.4452   $219.45   $219.45   $219.40  
  Purchase #5 $104.76   9.837%   $115.0652   $115.07   $115.05   $115.10  
  Purchase #6 $232.52   6.295%   $247.1571   $247.16   $247.15   $247.20  
  Purchase #7 $211.95   12.595%   $238.6451   $238.65   $238.65   $238.60  
  7 purchase totals = $891.78       $973.7815   $973.81   $973.75   $973.80