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 $18.45   12.229%   $20.7063   $20.71   $20.70   $20.70  
  Purchase #2 $158.99   13.192%   $179.964   $179.96   $179.95   $180.00  
  Purchase #3 $113.26   5.531%   $119.5244   $119.52   $119.50   $119.50  
  Purchase #4 $170.72   12.267%   $191.6622   $191.66   $191.65   $191.70  
  Purchase #5 $34.81   8.603%   $37.8047   $37.80   $37.80   $37.80  
  Purchase #6 $175.77   6.296%   $186.8365   $186.84   $186.85   $186.80  
  Purchase #7 $99.31   5.660%   $104.9309   $104.93   $104.95   $104.90  
  7 purchase totals = $771.31       $841.4290   $841.42   $841.40   $841.40