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 $120.67   6.142%   $128.0816   $128.08   $128.10   $128.10  
  Purchase #2 $166.47   8.238%   $180.1838   $180.18   $180.20   $180.20  
  Purchase #3 $26.44   13.813%   $30.0922   $30.09   $30.10   $30.10  
  Purchase #4 $119.58   12.150%   $134.109   $134.11   $134.10   $134.10  
  Purchase #5 $62.87   3.550%   $65.1019   $65.10   $65.10   $65.10  
  Purchase #6 $97.98   4.331%   $102.2235   $102.22   $102.20   $102.20  
  Purchase #7 $19.76   7.482%   $21.2384   $21.24   $21.25   $21.20  
  7 purchase totals = $613.77       $661.0304   $661.02   $661.05   $661.00