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 $244.69   12.289%   $274.76   $274.76   $274.75   $274.80  
  Purchase #2 $111.40   12.167%   $124.954   $124.95   $124.95   $125.00  
  Purchase #3 $192.26   10.686%   $212.8049   $212.80   $212.80   $212.80  
  Purchase #4 $37.97   11.650%   $42.3935   $42.39   $42.40   $42.40  
  Purchase #5 $196.86   7.261%   $211.154   $211.15   $211.15   $211.20  
  Purchase #6 $230.37   5.297%   $242.5727   $242.57   $242.55   $242.60  
  Purchase #7 $123.65   13.369%   $140.1808   $140.18   $140.20   $140.20  
  7 purchase totals = $1,137.20       $1,248.8199   $1,248.80   $1,248.80   $1,249.00