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 $234.33   13.417%   $265.7701   $265.77   $265.75   $265.80  
  Purchase #2 $134.34   6.412%   $142.9539   $142.95   $142.95   $143.00  
  Purchase #3 $222.32   8.757%   $241.7886   $241.79   $241.80   $241.80  
  Purchase #4 $203.50   10.365%   $224.5928   $224.59   $224.60   $224.60  
  Purchase #5 $24.99   12.368%   $28.0808   $28.08   $28.10   $28.10  
  Purchase #6 $199.40   3.607%   $206.5924   $206.59   $206.60   $206.60  
  Purchase #7 $132.81   10.178%   $146.3274   $146.33   $146.35   $146.30  
  7 purchase totals = $1,151.69       $1,256.1060   $1,256.10   $1,256.15   $1,256.20