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 $30.63   13.463%   $34.7537   $34.75   $34.75   $34.80  
  Purchase #2 $19.59   4.676%   $20.506   $20.51   $20.50   $20.50  
  Purchase #3 $56.80   6.747%   $60.6323   $60.63   $60.65   $60.60  
  Purchase #4 $83.49   7.251%   $89.5439   $89.54   $89.55   $89.50  
  Purchase #5 $246.25   8.428%   $267.004   $267.00   $267.00   $267.00  
  Purchase #6 $131.68   11.680%   $147.0602   $147.06   $147.05   $147.10  
  Purchase #7 $79.54   7.699%   $85.6638   $85.66   $85.65   $85.70  
  7 purchase totals = $647.98       $705.1639   $705.15   $705.15   $705.20