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 $142.71   3.202%   $147.2796   $147.28   $147.30   $147.30  
  Purchase #2 $21.57   9.830%   $23.6903   $23.69   $23.70   $23.70  
  Purchase #3 $95.33   5.501%   $100.5741   $100.57   $100.55   $100.60  
  Purchase #4 $153.50   8.479%   $166.5153   $166.52   $166.50   $166.50  
  Purchase #5 $25.77   7.847%   $27.7922   $27.79   $27.80   $27.80  
  Purchase #6 $146.49   12.123%   $164.249   $164.25   $164.25   $164.20  
  Purchase #7 $207.35   13.345%   $235.0209   $235.02   $235.00   $235.00  
  7 purchase totals = $792.72       $865.1214   $865.12   $865.10   $865.10