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 $246.76   10.764%   $273.3212   $273.32   $273.30   $273.30  
  Purchase #2 $213.36   7.484%   $229.3279   $229.33   $229.35   $229.30  
  Purchase #3 $89.52   3.345%   $92.5144   $92.51   $92.50   $92.50  
  Purchase #4 $198.74   10.301%   $219.2122   $219.21   $219.20   $219.20  
  Purchase #5 $191.73   12.532%   $215.7576   $215.76   $215.75   $215.80  
  Purchase #6 $206.66   3.332%   $213.5459   $213.55   $213.55   $213.50  
  Purchase #7 $27.81   4.335%   $29.0156   $29.02   $29.00   $29.00  
  7 purchase totals = $1,174.58       $1,272.6948   $1,272.70   $1,272.65   $1,272.60