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 $246.79   12.851%   $278.505   $278.51   $278.50   $278.50  
  Purchase #2 $61.64   3.471%   $63.7795   $63.78   $63.80   $63.80  
  Purchase #3 $238.93   4.356%   $249.3378   $249.34   $249.35   $249.30  
  Purchase #4 $188.54   12.433%   $211.9812   $211.98   $212.00   $212.00  
  Purchase #5 $91.34   6.239%   $97.0387   $97.04   $97.05   $97.00  
  Purchase #6 $142.47   6.391%   $151.5753   $151.58   $151.60   $151.60  
  Purchase #7 $208.38   11.396%   $232.127   $232.13   $232.15   $232.10  
  7 purchase totals = $1,178.09       $1,284.3445   $1,284.36   $1,284.45   $1,284.30