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 $6.90   9.116%   $7.529   $7.53   $7.55   $7.50  
  Purchase #2 $137.74   7.308%   $147.806   $147.81   $147.80   $147.80  
  Purchase #3 $200.61   9.165%   $218.9959   $219.00   $219.00   $219.00  
  Purchase #4 $166.42   4.631%   $174.1269   $174.13   $174.15   $174.10  
  Purchase #5 $152.82   4.431%   $159.5915   $159.59   $159.60   $159.60  
  Purchase #6 $244.59   4.783%   $256.2887   $256.29   $256.30   $256.30  
  Purchase #7 $148.42   5.838%   $157.0848   $157.08   $157.10   $157.10  
  7 purchase totals = $1,057.50       $1,121.4228   $1,121.43   $1,121.50   $1,121.40