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 $28.58   8.426%   $30.9882   $30.99   $31.00   $31.00  
  Purchase #2 $67.67   8.513%   $73.4307   $73.43   $73.45   $73.40  
  Purchase #3 $246.49   12.768%   $277.9618   $277.96   $277.95   $278.00  
  Purchase #4 $24.51   6.799%   $26.1764   $26.18   $26.20   $26.20  
  Purchase #5 $36.33   9.178%   $39.6644   $39.66   $39.65   $39.70  
  Purchase #6 $71.84   10.501%   $79.3839   $79.38   $79.40   $79.40  
  Purchase #7 $148.36   11.686%   $165.6973   $165.70   $165.70   $165.70  
  7 purchase totals = $623.78       $693.3027   $693.30   $693.35   $693.40