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 $243.55   12.174%   $273.1998   $273.20   $273.20   $273.20  
  Purchase #2 $203.29   11.274%   $226.2089   $226.21   $226.20   $226.20  
  Purchase #3 $56.79   7.733%   $61.1816   $61.18   $61.20   $61.20  
  Purchase #4 $166.78   10.754%   $184.7155   $184.72   $184.70   $184.70  
  Purchase #5 $215.27   13.297%   $243.8945   $243.89   $243.90   $243.90  
  Purchase #6 $123.47   8.341%   $133.7686   $133.77   $133.75   $133.80  
  Purchase #7 $231.91   7.342%   $248.9368   $248.94   $248.95   $248.90  
  7 purchase totals = $1,241.06       $1,371.9057   $1,371.91   $1,371.90   $1,371.90