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 $77.93   3.675%   $80.7939   $80.79   $80.80   $80.80  
  Purchase #2 $44.79   12.383%   $50.3363   $50.34   $50.35   $50.30  
  Purchase #3 $41.72   12.158%   $46.7923   $46.79   $46.80   $46.80  
  Purchase #4 $38.98   10.607%   $43.1146   $43.11   $43.10   $43.10  
  Purchase #5 $241.51   12.860%   $272.5682   $272.57   $272.55   $272.60  
  Purchase #6 $90.26   10.147%   $99.4187   $99.42   $99.40   $99.40  
  Purchase #7 $76.58   6.352%   $81.4444   $81.44   $81.45   $81.40  
  7 purchase totals = $611.77       $674.4684   $674.46   $674.45   $674.40