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 $9.34   4.810%   $9.7893   $9.79   $9.80   $9.80  
  Purchase #2 $98.69   6.216%   $104.8246   $104.82   $104.80   $104.80  
  Purchase #3 $134.67   9.273%   $147.1579   $147.16   $147.15   $147.20  
  Purchase #4 $82.91   12.317%   $93.122   $93.12   $93.10   $93.10  
  Purchase #5 $114.52   9.765%   $125.7029   $125.70   $125.70   $125.70  
  Purchase #6 $131.48   9.564%   $144.0547   $144.05   $144.05   $144.10  
  Purchase #7 $83.99   9.515%   $91.9816   $91.98   $92.00   $92.00  
  7 purchase totals = $655.60       $716.6330   $716.62   $716.60   $716.70