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 $238.74   10.417%   $263.6095   $263.61   $263.60   $263.60  
  Purchase #2 $169.30   5.562%   $178.7165   $178.72   $178.70   $178.70  
  Purchase #3 $126.36   6.832%   $134.9929   $134.99   $135.00   $135.00  
  Purchase #4 $10.65   8.120%   $11.5148   $11.51   $11.50   $11.50  
  Purchase #5 $33.29   6.254%   $35.372   $35.37   $35.35   $35.40  
  Purchase #6 $90.35   11.203%   $100.4719   $100.47   $100.45   $100.50  
  Purchase #7 $245.37   11.274%   $273.033   $273.03   $273.05   $273.00  
  7 purchase totals = $914.06       $997.7106   $997.70   $997.65   $997.70