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 $165.64   3.521%   $171.4722   $171.47   $171.45   $171.50  
  Purchase #2 $107.57   9.225%   $117.4933   $117.49   $117.50   $117.50  
  Purchase #3 $84.29   12.780%   $95.0623   $95.06   $95.05   $95.10  
  Purchase #4 $34.76   3.566%   $35.9995   $36.00   $36.00   $36.00  
  Purchase #5 $71.26   3.815%   $73.9786   $73.98   $74.00   $74.00  
  Purchase #6 $28.58   4.483%   $29.8612   $29.86   $29.85   $29.90  
  Purchase #7 $141.63   12.218%   $158.9344   $158.93   $158.95   $158.90  
  7 purchase totals = $633.73       $682.8015   $682.79   $682.80   $682.90