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 $9.90   9.774%   $10.8676   $10.87   $10.85   $10.90  
  Purchase #2 $85.62   12.760%   $96.5451   $96.55   $96.55   $96.50  
  Purchase #3 $5.84   11.616%   $6.5184   $6.52   $6.50   $6.50  
  Purchase #4 $74.89   7.123%   $80.2244   $80.22   $80.20   $80.20  
  Purchase #5 $84.38   5.625%   $89.1264   $89.13   $89.15   $89.10  
  Purchase #6 $204.60   6.633%   $218.1711   $218.17   $218.15   $218.20  
  Purchase #7 $42.87   4.765%   $44.9128   $44.91   $44.90   $44.90  
  7 purchase totals = $508.10       $546.3658   $546.37   $546.30   $546.30