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 $114.65   5.529%   $120.989   $120.99   $121.00   $121.00  
  Purchase #2 $49.77   11.217%   $55.3527   $55.35   $55.35   $55.40  
  Purchase #3 $111.81   6.164%   $118.702   $118.70   $118.70   $118.70  
  Purchase #4 $40.38   10.449%   $44.5993   $44.60   $44.60   $44.60  
  Purchase #5 $145.91   9.107%   $159.198   $159.20   $159.20   $159.20  
  Purchase #6 $211.53   5.708%   $223.6041   $223.60   $223.60   $223.60  
  Purchase #7 $87.85   9.214%   $95.9445   $95.94   $95.95   $95.90  
  7 purchase totals = $761.90       $818.3896   $818.38   $818.40   $818.40