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 $177.75   6.817%   $189.8672   $189.87   $189.85   $189.90  
  Purchase #2 $194.82   8.256%   $210.9043   $210.90   $210.90   $210.90  
  Purchase #3 $126.96   10.149%   $139.8452   $139.85   $139.85   $139.80  
  Purchase #4 $223.95   9.854%   $246.018   $246.02   $246.00   $246.00  
  Purchase #5 $63.27   11.286%   $70.4107   $70.41   $70.40   $70.40  
  Purchase #6 $30.63   3.719%   $31.7691   $31.77   $31.75   $31.80  
  Purchase #7 $36.47   7.313%   $39.1371   $39.14   $39.15   $39.10  
  7 purchase totals = $853.85       $927.9516   $927.96   $927.90   $927.90