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 $88.82   13.251%   $100.5895   $100.59   $100.60   $100.60  
  Purchase #2 $247.67   11.816%   $276.9347   $276.93   $276.95   $276.90  
  Purchase #3 $241.94   7.845%   $260.9202   $260.92   $260.90   $260.90  
  Purchase #4 $187.39   8.344%   $203.0258   $203.03   $203.05   $203.00  
  Purchase #5 $213.71   6.445%   $227.4836   $227.48   $227.50   $227.50  
  Purchase #6 $163.91   13.253%   $185.633   $185.63   $185.65   $185.60  
  Purchase #7 $138.27   5.380%   $145.7089   $145.71   $145.70   $145.70  
  7 purchase totals = $1,281.71       $1,400.2957   $1,400.29   $1,400.35   $1,400.20