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 $28.53   10.150%   $31.4258   $31.43   $31.45   $31.40  
  Purchase #2 $6.82   9.474%   $7.4661   $7.47   $7.45   $7.50  
  Purchase #3 $193.97   13.279%   $219.7273   $219.73   $219.75   $219.70  
  Purchase #4 $198.37   6.783%   $211.8254   $211.83   $211.85   $211.80  
  Purchase #5 $6.68   8.412%   $7.2419   $7.24   $7.25   $7.20  
  Purchase #6 $226.36   10.614%   $250.3859   $250.39   $250.40   $250.40  
  Purchase #7 $147.96   5.262%   $155.7457   $155.75   $155.75   $155.70  
  7 purchase totals = $808.69       $883.8181   $883.84   $883.90   $883.70