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 $109.30   11.703%   $122.0914   $122.09   $122.10   $122.10  
  Purchase #2 $164.28   8.275%   $177.8742   $177.87   $177.85   $177.90  
  Purchase #3 $136.69   12.721%   $154.0783   $154.08   $154.10   $154.10  
  Purchase #4 $5.71   13.210%   $6.4643   $6.46   $6.45   $6.50  
  Purchase #5 $27.45   3.809%   $28.4956   $28.50   $28.50   $28.50  
  Purchase #6 $133.80   7.470%   $143.7949   $143.79   $143.80   $143.80  
  Purchase #7 $29.61   6.281%   $31.4698   $31.47   $31.45   $31.50  
  7 purchase totals = $606.84       $664.2685   $664.26   $664.25   $664.40