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 $106.63   13.794%   $121.3385   $121.34   $121.35   $121.30  
  Purchase #2 $127.39   10.583%   $140.8717   $140.87   $140.85   $140.90  
  Purchase #3 $179.83   11.760%   $200.978   $200.98   $201.00   $201.00  
  Purchase #4 $41.77   9.234%   $45.627   $45.63   $45.65   $45.60  
  Purchase #5 $158.96   10.778%   $176.0927   $176.09   $176.10   $176.10  
  Purchase #6 $153.92   4.363%   $160.6355   $160.64   $160.65   $160.60  
  Purchase #7 $245.26   5.695%   $259.2276   $259.23   $259.25   $259.20  
  7 purchase totals = $1,013.76       $1,104.7710   $1,104.78   $1,104.85   $1,104.70