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 $245.72   8.309%   $266.1369   $266.14   $266.15   $266.10  
  Purchase #2 $219.47   6.326%   $233.3537   $233.35   $233.35   $233.40  
  Purchase #3 $34.49   9.109%   $37.6317   $37.63   $37.65   $37.60  
  Purchase #4 $166.31   3.513%   $172.1525   $172.15   $172.15   $172.20  
  Purchase #5 $149.45   4.799%   $156.6221   $156.62   $156.60   $156.60  
  Purchase #6 $143.76   4.289%   $149.9259   $149.93   $149.95   $149.90  
  Purchase #7 $5.79   4.598%   $6.0562   $6.06   $6.05   $6.10  
  7 purchase totals = $964.99       $1,021.8790   $1,021.88   $1,021.90   $1,021.90