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 $36.56   9.299%   $39.9597   $39.96   $39.95   $40.00  
  Purchase #2 $94.74   7.788%   $102.1184   $102.12   $102.10   $102.10  
  Purchase #3 $19.60   13.828%   $22.3103   $22.31   $22.30   $22.30  
  Purchase #4 $6.75   4.498%   $7.0536   $7.05   $7.05   $7.10  
  Purchase #5 $42.77   3.580%   $44.3012   $44.30   $44.30   $44.30  
  Purchase #6 $136.98   10.494%   $151.3547   $151.35   $151.35   $151.40  
  Purchase #7 $121.51   11.828%   $135.8822   $135.88   $135.90   $135.90  
  7 purchase totals = $458.91       $502.9801   $502.97   $502.95   $503.10