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 $246.81   3.751%   $256.0678   $256.07   $256.05   $256.10  
  Purchase #2 $122.62   4.272%   $127.8583   $127.86   $127.85   $127.90  
  Purchase #3 $78.37   6.479%   $83.4476   $83.45   $83.45   $83.40  
  Purchase #4 $23.47   12.326%   $26.3629   $26.36   $26.35   $26.40  
  Purchase #5 $186.89   7.645%   $201.1777   $201.18   $201.20   $201.20  
  Purchase #6 $61.28   6.232%   $65.099   $65.10   $65.10   $65.10  
  Purchase #7 $68.85   12.535%   $77.4803   $77.48   $77.50   $77.50  
  7 purchase totals = $788.29       $837.4936   $837.50   $837.50   $837.60