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 $6.54   4.415%   $6.8287   $6.83   $6.85   $6.80  
  Purchase #2 $87.84   12.447%   $98.7734   $98.77   $98.75   $98.80  
  Purchase #3 $85.53   13.630%   $97.1877   $97.19   $97.20   $97.20  
  Purchase #4 $154.94   7.128%   $165.9841   $165.98   $166.00   $166.00  
  Purchase #5 $182.85   5.118%   $192.2083   $192.21   $192.20   $192.20  
  Purchase #6 $34.84   6.704%   $37.1757   $37.18   $37.20   $37.20  
  Purchase #7 $46.31   7.653%   $49.8541   $49.85   $49.85   $49.90  
  7 purchase totals = $598.85       $648.0120   $648.01   $648.05   $648.10