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 $85.90   4.700%   $89.9373   $89.94   $89.95   $89.90  
  Purchase #2 $180.99   5.741%   $191.3806   $191.38   $191.40   $191.40  
  Purchase #3 $10.82   10.728%   $11.9808   $11.98   $12.00   $12.00  
  Purchase #4 $114.76   12.342%   $128.9237   $128.92   $128.90   $128.90  
  Purchase #5 $67.77   11.828%   $75.7858   $75.79   $75.80   $75.80  
  Purchase #6 $106.91   10.421%   $118.0511   $118.05   $118.05   $118.10  
  Purchase #7 $23.41   12.620%   $26.3643   $26.36   $26.35   $26.40  
  7 purchase totals = $590.56       $642.4236   $642.42   $642.45   $642.50