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 $36.40   9.508%   $39.8609   $39.86   $39.85   $39.90  
  Purchase #2 $84.40   8.273%   $91.3824   $91.38   $91.40   $91.40  
  Purchase #3 $247.42   6.407%   $263.2722   $263.27   $263.25   $263.30  
  Purchase #4 $177.77   13.647%   $202.0303   $202.03   $202.05   $202.00  
  Purchase #5 $55.66   13.183%   $62.9977   $63.00   $63.00   $63.00  
  Purchase #6 $43.90   3.756%   $45.5489   $45.55   $45.55   $45.50  
  Purchase #7 $6.67   8.860%   $7.261   $7.26   $7.25   $7.30  
  7 purchase totals = $652.22       $712.3534   $712.35   $712.35   $712.40