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 $28.72   9.645%   $31.49   $31.49   $31.50   $31.50  
  Purchase #2 $79.34   11.761%   $88.6712   $88.67   $88.65   $88.70  
  Purchase #3 $90.25   12.859%   $101.8552   $101.86   $101.85   $101.90  
  Purchase #4 $216.54   13.432%   $245.6257   $245.63   $245.65   $245.60  
  Purchase #5 $118.85   10.372%   $131.1771   $131.18   $131.20   $131.20  
  Purchase #6 $124.42   6.494%   $132.4998   $132.50   $132.50   $132.50  
  Purchase #7 $110.29   5.543%   $116.4034   $116.40   $116.40   $116.40  
  7 purchase totals = $768.41       $847.7224   $847.73   $847.75   $847.80