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 $244.45   11.852%   $273.4222   $273.42   $273.40   $273.40  
  Purchase #2 $157.43   11.133%   $174.9567   $174.96   $174.95   $175.00  
  Purchase #3 $228.77   8.852%   $249.0207   $249.02   $249.00   $249.00  
  Purchase #4 $61.34   11.181%   $68.1984   $68.20   $68.20   $68.20  
  Purchase #5 $182.78   10.841%   $202.5952   $202.60   $202.60   $202.60  
  Purchase #6 $191.95   11.606%   $214.2277   $214.23   $214.25   $214.20  
  Purchase #7 $82.77   12.822%   $93.3828   $93.38   $93.40   $93.40  
  7 purchase totals = $1,149.49       $1,275.8037   $1,275.81   $1,275.80   $1,275.80