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 $246.95   11.262%   $274.7615   $274.76   $274.75   $274.80  
  Purchase #2 $198.28   10.502%   $219.1034   $219.10   $219.10   $219.10  
  Purchase #3 $45.39   5.282%   $47.7875   $47.79   $47.80   $47.80  
  Purchase #4 $127.99   4.441%   $133.674   $133.67   $133.65   $133.70  
  Purchase #5 $171.51   7.323%   $184.0697   $184.07   $184.05   $184.10  
  Purchase #6 $64.65   5.703%   $68.337   $68.34   $68.35   $68.30  
  Purchase #7 $57.28   6.539%   $61.0255   $61.03   $61.05   $61.00  
  7 purchase totals = $912.05       $988.7586   $988.76   $988.75   $988.80