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 $7.80   8.429%   $8.4575   $8.46   $8.45   $8.50  
  Purchase #2 $156.48   8.142%   $169.2206   $169.22   $169.20   $169.20  
  Purchase #3 $99.36   12.864%   $112.1417   $112.14   $112.15   $112.10  
  Purchase #4 $60.47   6.204%   $64.2216   $64.22   $64.20   $64.20  
  Purchase #5 $197.49   7.396%   $212.0964   $212.10   $212.10   $212.10  
  Purchase #6 $189.52   13.676%   $215.4388   $215.44   $215.45   $215.40  
  Purchase #7 $134.84   11.248%   $150.0068   $150.01   $150.00   $150.00  
  7 purchase totals = $845.96       $931.5834   $931.59   $931.55   $931.50