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 $28.49   10.640%   $31.5213   $31.52   $31.50   $31.50  
  Purchase #2 $31.60   4.541%   $33.035   $33.04   $33.05   $33.00  
  Purchase #3 $197.70   4.765%   $207.1204   $207.12   $207.10   $207.10  
  Purchase #4 $178.76   3.353%   $184.7538   $184.75   $184.75   $184.80  
  Purchase #5 $134.67   11.735%   $150.4735   $150.47   $150.45   $150.50  
  Purchase #6 $27.84   12.414%   $31.2961   $31.30   $31.30   $31.30  
  Purchase #7 $38.88   7.631%   $41.8469   $41.85   $41.85   $41.80  
  7 purchase totals = $637.94       $680.0470   $680.05   $680.00   $680.00