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 $227.74   10.344%   $251.2974   $251.30   $251.30   $251.30  
  Purchase #2 $124.63   7.138%   $133.5261   $133.53   $133.55   $133.50  
  Purchase #3 $176.70   11.166%   $196.4303   $196.43   $196.45   $196.40  
  Purchase #4 $110.57   10.637%   $122.3313   $122.33   $122.35   $122.30  
  Purchase #5 $23.83   9.485%   $26.0903   $26.09   $26.10   $26.10  
  Purchase #6 $5.37   3.141%   $5.5387   $5.54   $5.55   $5.50  
  Purchase #7 $31.93   6.540%   $34.0182   $34.02   $34.00   $34.00  
  7 purchase totals = $700.77       $769.2323   $769.24   $769.30   $769.10