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 $74.50   9.354%   $81.4687   $81.47   $81.45   $81.50  
  Purchase #2 $135.75   9.122%   $148.1331   $148.13   $148.15   $148.10  
  Purchase #3 $154.92   3.730%   $160.6985   $160.70   $160.70   $160.70  
  Purchase #4 $94.98   5.757%   $100.448   $100.45   $100.45   $100.40  
  Purchase #5 $159.96   8.125%   $172.9568   $172.96   $172.95   $173.00  
  Purchase #6 $70.89   13.476%   $80.4431   $80.44   $80.45   $80.40  
  Purchase #7 $151.55   12.404%   $170.3483   $170.35   $170.35   $170.30  
  7 purchase totals = $842.55       $914.4965   $914.50   $914.50   $914.40