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 $238.35   12.873%   $269.0328   $269.03   $269.05   $269.00  
  Purchase #2 $69.72   4.685%   $72.9864   $72.99   $73.00   $73.00  
  Purchase #3 $227.91   12.447%   $256.278   $256.28   $256.30   $256.30  
  Purchase #4 $29.39   11.501%   $32.7701   $32.77   $32.75   $32.80  
  Purchase #5 $89.86   6.442%   $95.6488   $95.65   $95.65   $95.60  
  Purchase #6 $120.56   6.312%   $128.1697   $128.17   $128.15   $128.20  
  Purchase #7 $69.25   11.161%   $76.979   $76.98   $77.00   $77.00  
  7 purchase totals = $845.04       $931.8648   $931.87   $931.90   $931.90