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 $79.97   9.408%   $87.4936   $87.49   $87.50   $87.50  
  Purchase #2 $152.96   4.490%   $159.8279   $159.83   $159.85   $159.80  
  Purchase #3 $250.56   3.702%   $259.8357   $259.84   $259.85   $259.80  
  Purchase #4 $75.49   10.376%   $83.3228   $83.32   $83.30   $83.30  
  Purchase #5 $25.84   11.529%   $28.8191   $28.82   $28.80   $28.80  
  Purchase #6 $103.76   5.822%   $109.8009   $109.80   $109.80   $109.80  
  Purchase #7 $149.57   5.348%   $157.569   $157.57   $157.55   $157.60  
  7 purchase totals = $838.15       $886.6690   $886.67   $886.65   $886.60