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 $56.83   3.707%   $58.9367   $58.94   $58.95   $58.90  
  Purchase #2 $230.55   11.686%   $257.4921   $257.49   $257.50   $257.50  
  Purchase #3 $144.62   5.414%   $152.4497   $152.45   $152.45   $152.40  
  Purchase #4 $162.97   13.366%   $184.7526   $184.75   $184.75   $184.80  
  Purchase #5 $244.90   7.839%   $264.0977   $264.10   $264.10   $264.10  
  Purchase #6 $59.58   11.278%   $66.2994   $66.30   $66.30   $66.30  
  Purchase #7 $148.79   3.673%   $154.2551   $154.26   $154.25   $154.30  
  7 purchase totals = $1,048.24       $1,138.2833   $1,138.29   $1,138.30   $1,138.30