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 $31.30   10.850%   $34.6961   $34.70   $34.70   $34.70  
  Purchase #2 $102.50   5.680%   $108.322   $108.32   $108.30   $108.30  
  Purchase #3 $98.79   13.171%   $111.8016   $111.80   $111.80   $111.80  
  Purchase #4 $106.71   5.730%   $112.8245   $112.82   $112.80   $112.80  
  Purchase #5 $90.75   6.528%   $96.6742   $96.67   $96.65   $96.70  
  Purchase #6 $29.95   7.529%   $32.2049   $32.20   $32.20   $32.20  
  Purchase #7 $135.92   4.359%   $141.8448   $141.84   $141.85   $141.80  
  7 purchase totals = $595.92       $638.3681   $638.35   $638.30   $638.30