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 $250.53   3.364%   $258.9578   $258.96   $258.95   $259.00  
  Purchase #2 $189.85   6.169%   $201.5618   $201.56   $201.55   $201.60  
  Purchase #3 $59.54   3.344%   $61.531   $61.53   $61.55   $61.50  
  Purchase #4 $43.66   7.117%   $46.7673   $46.77   $46.75   $46.80  
  Purchase #5 $250.76   7.350%   $269.1909   $269.19   $269.20   $269.20  
  Purchase #6 $54.84   10.227%   $60.4485   $60.45   $60.45   $60.40  
  Purchase #7 $59.33   9.353%   $64.8791   $64.88   $64.90   $64.90  
  7 purchase totals = $908.51       $963.3364   $963.34   $963.35   $963.40