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 $17.75   3.539%   $18.3782   $18.38   $18.40   $18.40  
  Purchase #2 $28.85   10.448%   $31.8642   $31.86   $31.85   $31.90  
  Purchase #3 $33.49   9.739%   $36.7516   $36.75   $36.75   $36.80  
  Purchase #4 $53.31   13.137%   $60.3133   $60.31   $60.30   $60.30  
  Purchase #5 $34.98   4.115%   $36.4194   $36.42   $36.40   $36.40  
  Purchase #6 $35.49   4.154%   $36.9643   $36.96   $36.95   $37.00  
  Purchase #7 $127.83   10.739%   $141.5577   $141.56   $141.55   $141.60  
  7 purchase totals = $331.70       $362.2487   $362.24   $362.20   $362.40