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 $28.30   7.841%   $30.519   $30.52   $30.50   $30.50  
  Purchase #2 $174.50   8.482%   $189.3011   $189.30   $189.30   $189.30  
  Purchase #3 $211.27   4.585%   $220.9567   $220.96   $220.95   $221.00  
  Purchase #4 $247.43   13.163%   $279.9992   $280.00   $280.00   $280.00  
  Purchase #5 $172.95   8.445%   $187.5556   $187.56   $187.55   $187.60  
  Purchase #6 $72.39   4.651%   $75.7569   $75.76   $75.75   $75.80  
  Purchase #7 $98.34   6.847%   $105.0733   $105.07   $105.05   $105.10  
  7 purchase totals = $1,005.18       $1,089.1618   $1,089.17   $1,089.10   $1,089.30