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 $180.36   13.222%   $204.2072   $204.21   $204.20   $204.20  
  Purchase #2 $211.88   9.522%   $232.0552   $232.06   $232.05   $232.10  
  Purchase #3 $175.39   11.492%   $195.5458   $195.55   $195.55   $195.50  
  Purchase #4 $44.82   8.221%   $48.5047   $48.50   $48.50   $48.50  
  Purchase #5 $224.37   6.526%   $239.0124   $239.01   $239.00   $239.00  
  Purchase #6 $229.56   9.664%   $251.7447   $251.74   $251.75   $251.70  
  Purchase #7 $79.99   7.661%   $86.118   $86.12   $86.10   $86.10  
  7 purchase totals = $1,146.37       $1,257.1880   $1,257.19   $1,257.15   $1,257.10