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 $56.79   12.834%   $64.0784   $64.08   $64.10   $64.10  
  Purchase #2 $25.82   5.290%   $27.1859   $27.19   $27.20   $27.20  
  Purchase #3 $61.95   12.817%   $69.8901   $69.89   $69.90   $69.90  
  Purchase #4 $26.75   8.673%   $29.07   $29.07   $29.05   $29.10  
  Purchase #5 $9.77   10.768%   $10.822   $10.82   $10.80   $10.80  
  Purchase #6 $227.59   6.146%   $241.5777   $241.58   $241.60   $241.60  
  Purchase #7 $224.31   12.138%   $251.5367   $251.54   $251.55   $251.50  
  7 purchase totals = $632.98       $694.1608   $694.17   $694.20   $694.20