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 $6.57   3.338%   $6.7893   $6.79   $6.80   $6.80  
  Purchase #2 $128.49   10.242%   $141.6499   $141.65   $141.65   $141.60  
  Purchase #3 $87.79   4.309%   $91.5729   $91.57   $91.55   $91.60  
  Purchase #4 $104.70   11.258%   $116.4871   $116.49   $116.50   $116.50  
  Purchase #5 $18.84   12.679%   $21.2287   $21.23   $21.25   $21.20  
  Purchase #6 $236.98   7.102%   $253.8103   $253.81   $253.80   $253.80  
  Purchase #7 $28.93   9.222%   $31.5979   $31.60   $31.60   $31.60  
  7 purchase totals = $612.30       $663.1361   $663.14   $663.15   $663.10