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 $106.98   13.144%   $121.0415   $121.04   $121.05   $121.00  
  Purchase #2 $156.89   4.525%   $163.9893   $163.99   $164.00   $164.00  
  Purchase #3 $46.53   12.626%   $52.4049   $52.40   $52.40   $52.40  
  Purchase #4 $191.57   5.380%   $201.8765   $201.88   $201.90   $201.90  
  Purchase #5 $154.42   11.794%   $172.6323   $172.63   $172.65   $172.60  
  Purchase #6 $200.30   9.545%   $219.4186   $219.42   $219.40   $219.40  
  Purchase #7 $120.93   5.505%   $127.5872   $127.59   $127.60   $127.60  
  7 purchase totals = $977.62       $1,058.9503   $1,058.95   $1,059.00   $1,058.90