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 $6.50   13.447%   $7.3741   $7.37   $7.35   $7.40  
  Purchase #2 $84.95   6.121%   $90.1498   $90.15   $90.15   $90.10  
  Purchase #3 $9.82   3.559%   $10.1695   $10.17   $10.15   $10.20  
  Purchase #4 $74.35   10.654%   $82.2712   $82.27   $82.25   $82.30  
  Purchase #5 $121.38   10.183%   $133.7401   $133.74   $133.75   $133.70  
  Purchase #6 $227.98   7.547%   $245.1857   $245.19   $245.20   $245.20  
  Purchase #7 $165.98   8.421%   $179.9572   $179.96   $179.95   $180.00  
  7 purchase totals = $690.96       $748.8476   $748.85   $748.80   $748.90