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 $7.81   11.337%   $8.6954   $8.70   $8.70   $8.70  
  Purchase #2 $154.25   7.158%   $165.2912   $165.29   $165.30   $165.30  
  Purchase #3 $31.29   5.352%   $32.9646   $32.96   $32.95   $33.00  
  Purchase #4 $97.66   13.710%   $111.0492   $111.05   $111.05   $111.00  
  Purchase #5 $122.96   7.202%   $131.8156   $131.82   $131.80   $131.80  
  Purchase #6 $90.90   7.450%   $97.6721   $97.67   $97.65   $97.70  
  Purchase #7 $37.88   5.201%   $39.8501   $39.85   $39.85   $39.90  
  7 purchase totals = $542.75       $587.3382   $587.34   $587.30   $587.40