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 $16.80   11.571%   $18.7439   $18.74   $18.75   $18.70  
  Purchase #2 $79.50   10.857%   $88.1313   $88.13   $88.15   $88.10  
  Purchase #3 $172.43   9.759%   $189.2574   $189.26   $189.25   $189.30  
  Purchase #4 $172.55   6.831%   $184.3369   $184.34   $184.35   $184.30  
  Purchase #5 $176.28   3.679%   $182.7653   $182.77   $182.75   $182.80  
  Purchase #6 $120.39   9.592%   $131.9378   $131.94   $131.95   $131.90  
  Purchase #7 $83.42   9.471%   $91.3207   $91.32   $91.30   $91.30  
  7 purchase totals = $821.37       $886.4933   $886.50   $886.50   $886.40