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 $52.79   12.122%   $59.1892   $59.19   $59.20   $59.20  
  Purchase #2 $224.64   10.293%   $247.7622   $247.76   $247.75   $247.80  
  Purchase #3 $38.41   3.166%   $39.6261   $39.63   $39.65   $39.60  
  Purchase #4 $230.52   5.492%   $243.1802   $243.18   $243.20   $243.20  
  Purchase #5 $36.77   13.868%   $41.8693   $41.87   $41.85   $41.90  
  Purchase #6 $167.81   5.133%   $176.4237   $176.42   $176.40   $176.40  
  Purchase #7 $33.85   3.454%   $35.0192   $35.02   $35.00   $35.00  
  7 purchase totals = $784.79       $843.0699   $843.07   $843.05   $843.10