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 $19.31   13.207%   $21.8603   $21.86   $21.85   $21.90  
  Purchase #2 $28.74   7.115%   $30.7849   $30.78   $30.80   $30.80  
  Purchase #3 $15.69   3.113%   $16.1784   $16.18   $16.20   $16.20  
  Purchase #4 $203.79   13.436%   $231.1712   $231.17   $231.15   $231.20  
  Purchase #5 $37.76   12.413%   $42.4471   $42.45   $42.45   $42.40  
  Purchase #6 $31.39   3.458%   $32.4755   $32.48   $32.50   $32.50  
  Purchase #7 $241.99   13.199%   $273.9303   $273.93   $273.95   $273.90  
  7 purchase totals = $578.67       $648.8477   $648.85   $648.90   $648.90