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 $236.60   12.542%   $266.2744   $266.27   $266.25   $266.30  
  Purchase #2 $183.99   13.185%   $208.2491   $208.25   $208.25   $208.20  
  Purchase #3 $145.56   11.802%   $162.739   $162.74   $162.75   $162.70  
  Purchase #4 $181.31   11.417%   $202.0102   $202.01   $202.00   $202.00  
  Purchase #5 $226.54   12.135%   $254.0306   $254.03   $254.05   $254.00  
  Purchase #6 $98.60   11.650%   $110.0869   $110.09   $110.10   $110.10  
  Purchase #7 $225.92   12.682%   $254.5712   $254.57   $254.55   $254.60  
  7 purchase totals = $1,298.52       $1,457.9614   $1,457.96   $1,457.95   $1,457.90