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 $136.40   8.418%   $147.8822   $147.88   $147.90   $147.90  
  Purchase #2 $28.45   9.796%   $31.237   $31.24   $31.25   $31.20  
  Purchase #3 $152.84   6.528%   $162.8174   $162.82   $162.80   $162.80  
  Purchase #4 $12.98   9.447%   $14.2062   $14.21   $14.20   $14.20  
  Purchase #5 $60.46   8.829%   $65.798   $65.80   $65.80   $65.80  
  Purchase #6 $152.77   7.513%   $164.2476   $164.25   $164.25   $164.20  
  Purchase #7 $54.91   12.344%   $61.6881   $61.69   $61.70   $61.70  
  7 purchase totals = $598.81       $647.8765   $647.89   $647.90   $647.80