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 $63.50   4.471%   $66.3391   $66.34   $66.35   $66.30  
  Purchase #2 $17.38   7.318%   $18.6519   $18.65   $18.65   $18.70  
  Purchase #3 $226.83   4.601%   $237.2664   $237.27   $237.25   $237.30  
  Purchase #4 $101.79   13.495%   $115.5266   $115.53   $115.55   $115.50  
  Purchase #5 $56.85   6.866%   $60.7533   $60.75   $60.75   $60.80  
  Purchase #6 $91.27   8.753%   $99.2589   $99.26   $99.25   $99.30  
  Purchase #7 $220.82   9.197%   $241.1288   $241.13   $241.15   $241.10  
  7 purchase totals = $778.44       $838.9250   $838.93   $838.95   $839.00