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 $120.83   4.271%   $125.9906   $125.99   $126.00   $126.00  
  Purchase #2 $62.74   8.790%   $68.2548   $68.25   $68.25   $68.30  
  Purchase #3 $240.29   4.178%   $250.3293   $250.33   $250.35   $250.30  
  Purchase #4 $219.25   6.428%   $233.3434   $233.34   $233.35   $233.30  
  Purchase #5 $127.76   6.839%   $136.4975   $136.50   $136.50   $136.50  
  Purchase #6 $56.25   8.777%   $61.1871   $61.19   $61.20   $61.20  
  Purchase #7 $104.30   3.461%   $107.9098   $107.91   $107.90   $107.90  
  7 purchase totals = $931.42       $983.5125   $983.51   $983.55   $983.50