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 $249.98   10.406%   $275.9929   $275.99   $276.00   $276.00  
  Purchase #2 $160.57   11.262%   $178.6534   $178.65   $178.65   $178.70  
  Purchase #3 $109.85   6.667%   $117.1737   $117.17   $117.15   $117.20  
  Purchase #4 $225.29   13.624%   $255.9835   $255.98   $256.00   $256.00  
  Purchase #5 $56.98   10.746%   $63.1031   $63.10   $63.10   $63.10  
  Purchase #6 $126.41   8.799%   $137.5328   $137.53   $137.55   $137.50  
  Purchase #7 $54.81   10.445%   $60.5349   $60.53   $60.55   $60.50  
  7 purchase totals = $983.89       $1,088.9743   $1,088.95   $1,089.00   $1,089.00