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 $206.91   13.400%   $234.6359   $234.64   $234.65   $234.60  
  Purchase #2 $26.86   7.188%   $28.7907   $28.79   $28.80   $28.80  
  Purchase #3 $10.28   13.694%   $11.6877   $11.69   $11.70   $11.70  
  Purchase #4 $200.86   13.360%   $227.6949   $227.69   $227.70   $227.70  
  Purchase #5 $214.35   12.671%   $241.5103   $241.51   $241.50   $241.50  
  Purchase #6 $135.82   10.414%   $149.9643   $149.96   $149.95   $150.00  
  Purchase #7 $45.58   5.796%   $48.2218   $48.22   $48.20   $48.20  
  7 purchase totals = $840.66       $942.5056   $942.50   $942.50   $942.50