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 $23.27   13.133%   $26.326   $26.33   $26.35   $26.30  
  Purchase #2 $243.31   5.178%   $255.9086   $255.91   $255.90   $255.90  
  Purchase #3 $44.41   5.417%   $46.8157   $46.82   $46.80   $46.80  
  Purchase #4 $34.80   5.741%   $36.7979   $36.80   $36.80   $36.80  
  Purchase #5 $58.45   3.355%   $60.411   $60.41   $60.40   $60.40  
  Purchase #6 $134.95   3.104%   $139.1388   $139.14   $139.15   $139.10  
  Purchase #7 $113.43   4.558%   $118.6001   $118.60   $118.60   $118.60  
  7 purchase totals = $652.62       $683.9981   $684.01   $684.00   $683.90