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 $31.79   7.750%   $34.2537   $34.25   $34.25   $34.30  
  Purchase #2 $193.62   12.711%   $218.231   $218.23   $218.25   $218.20  
  Purchase #3 $107.86   11.647%   $120.4225   $120.42   $120.40   $120.40  
  Purchase #4 $119.49   12.524%   $134.4549   $134.45   $134.45   $134.50  
  Purchase #5 $85.91   4.532%   $89.8034   $89.80   $89.80   $89.80  
  Purchase #6 $104.38   8.692%   $113.4527   $113.45   $113.45   $113.50  
  Purchase #7 $124.78   8.203%   $135.0157   $135.02   $135.00   $135.00  
  7 purchase totals = $767.83       $845.6339   $845.62   $845.60   $845.70