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 $28.42   9.426%   $31.0989   $31.10   $31.10   $31.10  
  Purchase #2 $95.51   5.403%   $100.6704   $100.67   $100.65   $100.70  
  Purchase #3 $86.99   7.810%   $93.7839   $93.78   $93.80   $93.80  
  Purchase #4 $192.62   3.353%   $199.0785   $199.08   $199.10   $199.10  
  Purchase #5 $7.60   10.182%   $8.3738   $8.37   $8.35   $8.40  
  Purchase #6 $90.93   10.829%   $100.7768   $100.78   $100.80   $100.80  
  Purchase #7 $124.84   12.505%   $140.4512   $140.45   $140.45   $140.50  
  7 purchase totals = $626.91       $674.2335   $674.23   $674.25   $674.40