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 $248.75   10.569%   $275.0404   $275.04   $275.05   $275.00  
  Purchase #2 $27.77   7.394%   $29.8233   $29.82   $29.80   $29.80  
  Purchase #3 $120.46   13.215%   $136.3788   $136.38   $136.40   $136.40  
  Purchase #4 $157.45   10.519%   $174.0122   $174.01   $174.00   $174.00  
  Purchase #5 $239.41   6.306%   $254.5072   $254.51   $254.50   $254.50  
  Purchase #6 $216.93   11.684%   $242.2761   $242.28   $242.30   $242.30  
  Purchase #7 $32.68   3.203%   $33.7267   $33.73   $33.75   $33.70  
  7 purchase totals = $1,043.45       $1,145.7647   $1,145.77   $1,145.80   $1,145.70