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 $166.39   8.511%   $180.5515   $180.55   $180.55   $180.60  
  Purchase #2 $31.45   6.176%   $33.3924   $33.39   $33.40   $33.40  
  Purchase #3 $15.77   10.306%   $17.3953   $17.40   $17.40   $17.40  
  Purchase #4 $38.96   6.672%   $41.5594   $41.56   $41.55   $41.60  
  Purchase #5 $30.79   7.342%   $33.0506   $33.05   $33.05   $33.10  
  Purchase #6 $30.99   3.219%   $31.9876   $31.99   $32.00   $32.00  
  Purchase #7 $78.79   4.394%   $82.252   $82.25   $82.25   $82.30  
  7 purchase totals = $393.14       $420.1888   $420.19   $420.20   $420.40