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 $28.62   4.815%   $29.9981   $30.00   $30.00   $30.00  
  Purchase #2 $187.80   12.368%   $211.0271   $211.03   $211.05   $211.00  
  Purchase #3 $34.31   5.694%   $36.2636   $36.26   $36.25   $36.30  
  Purchase #4 $135.28   9.215%   $147.7461   $147.75   $147.75   $147.70  
  Purchase #5 $16.79   12.212%   $18.8404   $18.84   $18.85   $18.80  
  Purchase #6 $17.73   12.803%   $20   $20.00   $20.00   $20.00  
  Purchase #7 $101.82   8.502%   $110.4767   $110.48   $110.50   $110.50  
  7 purchase totals = $522.35       $574.3520   $574.36   $574.40   $574.30