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 $31.56   5.758%   $33.3772   $33.38   $33.40   $33.40  
  Purchase #2 $188.48   3.275%   $194.6527   $194.65   $194.65   $194.70  
  Purchase #3 $109.80   10.239%   $121.0424   $121.04   $121.05   $121.00  
  Purchase #4 $7.98   5.254%   $8.3993   $8.40   $8.40   $8.40  
  Purchase #5 $22.99   5.573%   $24.2712   $24.27   $24.25   $24.30  
  Purchase #6 $6.98   9.521%   $7.6446   $7.64   $7.65   $7.60  
  Purchase #7 $9.82   8.816%   $10.6857   $10.69   $10.70   $10.70  
  7 purchase totals = $377.61       $400.0731   $400.07   $400.10   $400.10