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 $81.30   11.857%   $90.9397   $90.94   $90.95   $90.90  
  Purchase #2 $12.43   6.441%   $13.2306   $13.23   $13.25   $13.20  
  Purchase #3 $77.70   10.237%   $85.6541   $85.65   $85.65   $85.70  
  Purchase #4 $129.75   6.250%   $137.8594   $137.86   $137.85   $137.90  
  Purchase #5 $71.86   4.497%   $75.0915   $75.09   $75.10   $75.10  
  Purchase #6 $185.44   8.780%   $201.7216   $201.72   $201.70   $201.70  
  Purchase #7 $162.67   13.412%   $184.4873   $184.49   $184.50   $184.50  
  7 purchase totals = $721.15       $788.9842   $788.98   $789.00   $789.00