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 $227.39   13.463%   $258.0035   $258.00   $258.00   $258.00  
  Purchase #2 $176.45   10.370%   $194.7479   $194.75   $194.75   $194.70  
  Purchase #3 $222.48   7.786%   $239.8023   $239.80   $239.80   $239.80  
  Purchase #4 $156.67   13.130%   $177.2408   $177.24   $177.25   $177.20  
  Purchase #5 $151.50   10.871%   $167.9696   $167.97   $167.95   $168.00  
  Purchase #6 $161.39   11.164%   $179.4076   $179.41   $179.40   $179.40  
  Purchase #7 $177.90   7.831%   $191.8313   $191.83   $191.85   $191.80  
  7 purchase totals = $1,273.78       $1,409.0030   $1,409.00   $1,409.00   $1,408.90