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 $204.30   8.266%   $221.1874   $221.19   $221.20   $221.20  
  Purchase #2 $12.40   11.727%   $13.8541   $13.85   $13.85   $13.90  
  Purchase #3 $74.55   7.765%   $80.3388   $80.34   $80.35   $80.30  
  Purchase #4 $186.52   3.377%   $192.8188   $192.82   $192.80   $192.80  
  Purchase #5 $8.48   6.684%   $9.0468   $9.05   $9.05   $9.00  
  Purchase #6 $101.87   6.584%   $108.5771   $108.58   $108.60   $108.60  
  Purchase #7 $19.87   3.740%   $20.6131   $20.61   $20.60   $20.60  
  7 purchase totals = $607.99       $646.4361   $646.44   $646.45   $646.40