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 $18.62   13.292%   $21.095   $21.10   $21.10   $21.10  
  Purchase #2 $158.34   12.454%   $178.0597   $178.06   $178.05   $178.10  
  Purchase #3 $48.81   9.226%   $53.3132   $53.31   $53.30   $53.30  
  Purchase #4 $18.95   12.741%   $21.3644   $21.36   $21.35   $21.40  
  Purchase #5 $85.27   12.247%   $95.713   $95.71   $95.70   $95.70  
  Purchase #6 $72.34   7.712%   $77.9189   $77.92   $77.90   $77.90  
  Purchase #7 $5.93   10.254%   $6.5381   $6.54   $6.55   $6.50  
  7 purchase totals = $408.26       $454.0023   $454.00   $453.95   $454.00