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 $8.83   5.418%   $9.3084   $9.31   $9.30   $9.30  
  Purchase #2 $96.40   4.696%   $100.9269   $100.93   $100.95   $100.90  
  Purchase #3 $116.41   12.866%   $131.3873   $131.39   $131.40   $131.40  
  Purchase #4 $228.67   3.744%   $237.2314   $237.23   $237.25   $237.20  
  Purchase #5 $205.68   3.425%   $212.7245   $212.72   $212.70   $212.70  
  Purchase #6 $155.26   9.484%   $169.9849   $169.98   $170.00   $170.00  
  Purchase #7 $120.53   5.530%   $127.1953   $127.20   $127.20   $127.20  
  7 purchase totals = $931.78       $988.7587   $988.76   $988.80   $988.70