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 $123.54   11.225%   $137.4074   $137.41   $137.40   $137.40  
  Purchase #2 $77.39   4.371%   $80.7727   $80.77   $80.75   $80.80  
  Purchase #3 $97.57   11.567%   $108.8559   $108.86   $108.85   $108.90  
  Purchase #4 $180.81   9.512%   $198.0086   $198.01   $198.00   $198.00  
  Purchase #5 $152.67   11.362%   $170.0164   $170.02   $170.00   $170.00  
  Purchase #6 $24.54   10.211%   $27.0458   $27.05   $27.05   $27.00  
  Purchase #7 $105.45   10.237%   $116.2449   $116.24   $116.25   $116.20  
  7 purchase totals = $761.97       $838.3517   $838.36   $838.30   $838.30